session svm: tracking segment memory usage

1. segment manager would attempt to balance the usages across
   the segments, when it allocate fifos
2. the memory presure level is determined per fifo-segment
3. updated unit test
4. updated cli output for segments

Type: feature

Signed-off-by: Ryujiro Shibuya <ryujiro.shibuya@owmobility.com>
Change-Id: I2923f3e0a43dd919196a0cb2cd55e098fde6cf66

9 files changed, 1056 insertions, 32 deletions

diff --git a/src/plugins/unittest/CMakeLists.txt b/src/plugins/unittest/CMakeLists.txt
index a029e0e3259..e75b96ff265 100644
--- a/src/plugins/unittest/CMakeLists.txt
+++ b/src/plugins/unittest/CMakeLists.txt
@@ -36,6 +36,7 @@ add_vpp_plugin(unittest
   sparse_vec_test.c
   string_test.c
   svm_fifo_test.c
+  segment_manager_test.c
   tcp_test.c
   test_buffer.c
   unittest.c
diff --git a/src/plugins/unittest/segment_manager_test.c b/src/plugins/unittest/segment_manager_test.c
new file mode 100644
index 00000000000..8f362f40cb6
--- /dev/null
+++ b/src/plugins/unittest/segment_manager_test.c
@@ -0,0 +1,757 @@
+#include <vnet/vnet.h>
+#include <vnet/plugin/plugin.h>
+#include <vpp/app/version.h>
+#include <vnet/session/session.h>
+#include <vnet/session/segment_manager.h>
+#include <vnet/session/application.h>
+
+#define SEG_MGR_TEST_I(_cond, _comment, _args...)               \
+({                                                              \
+  int _evald = (_cond);                                         \
+  if (!(_evald)) {                                              \
+    fformat(stderr, "FAIL:%d: " _comment "\n",                  \
+            __LINE__, ##_args);                                 \
+  } else {                                                      \
+    fformat(stderr, "PASS:%d: " _comment "\n",                  \
+            __LINE__, ##_args);                                 \
+  }                                                             \
+  _evald;                                                       \
+})
+
+#define SEG_MGR_TEST(_cond, _comment, _args...)                 \
+{                                                               \
+    if (!SEG_MGR_TEST_I(_cond, _comment, ##_args)) {            \
+        return 1;                                               \
+    }                                                           \
+}
+
+#define ST_DBG(_comment, _args...)                              \
+    fformat(stderr,  _comment "\n",  ##_args);                  \
+
+#define SEGMENT_MANAGER_GET_INDEX_FROM_HANDLE(x) (x >> 32)
+
+/* dummy callback functions */
+static void
+dummy_session_reset_callback (session_t * s)
+{
+  clib_warning ("called...");
+}
+
+static int
+dummy_session_connected_callback (u32 app_index, u32 api_context,
+				  session_t * s, u8 is_fail)
+{
+  clib_warning ("called...");
+  return 0;
+}
+
+static int
+dummy_add_segment_callback (u32 client_index, u64 segment_handle)
+{
+  clib_warning ("called...");
+  return 0;
+}
+
+static int
+dummy_del_segment_callback (u32 client_index, u64 segment_handle)
+{
+  clib_warning ("called...");
+  return 0;
+}
+
+static void
+dummy_session_disconnect_callback (session_t * s)
+{
+  clib_warning ("called...");
+}
+
+static int
+dummy_session_accept_callback (session_t * s)
+{
+  clib_warning ("called...");
+  return 0;
+}
+
+static int
+dummy_server_rx_callback (session_t * s)
+{
+  clib_warning ("called...");
+  return -1;
+}
+
+/* *INDENT-OFF* */
+static session_cb_vft_t dummy_session_cbs = {
+  .session_reset_callback = dummy_session_reset_callback,
+  .session_connected_callback = dummy_session_connected_callback,
+  .session_accept_callback = dummy_session_accept_callback,
+  .session_disconnect_callback = dummy_session_disconnect_callback,
+  .builtin_app_rx_callback = dummy_server_rx_callback,
+  .add_segment_callback = dummy_add_segment_callback,
+  .del_segment_callback = dummy_del_segment_callback,
+};
+/* *INDENT-ON* */
+
+static char *states_str[] = {
+#define _(sym,str) str,
+  foreach_segment_mem_status
+#undef _
+};
+
+static u32 size_4KB = 4 << 10;
+static u32 size_8KB = 8 << 10;
+static u32 size_12KB = 12 << 10;
+static u32 size_16KB = 16 << 10;
+static u32 size_20KB = 20 << 10;
+static u32 size_32KB = 32 << 10;
+static u32 size_52KB = 52 << 10;
+static u32 size_64KB = 64 << 10;
+static u32 size_128KB = 128 << 10;
+static u32 size_1MB = 1 << 20;
+static u32 size_2MB = 2 << 20;
+
+
+static int
+segment_manager_test_pressure_1 (vlib_main_t * vm, unformat_input_t * input)
+{
+  int rv;
+  segment_manager_t *sm;
+  fifo_segment_t *fs0, *fs;
+  svm_fifo_t *rx_fifo, *tx_fifo;
+  uword app_seg_size = size_2MB;
+  u32 fifo_size = size_128KB;
+  u64 options[APP_OPTIONS_N_OPTIONS];
+  u8 data[size_128KB];
+
+  memset (&options, 0, sizeof (options));
+
+  vnet_app_attach_args_t attach_args = {
+    .api_client_index = ~0,
+    .options = options,
+    .namespace_id = 0,
+    .session_cb_vft = &dummy_session_cbs,
+    .name = format (0, "segment_manager_test_pressure_1"),
+  };
+
+  attach_args.options[APP_OPTIONS_SEGMENT_SIZE] = app_seg_size;
+  attach_args.options[APP_OPTIONS_FLAGS] = APP_OPTIONS_FLAGS_IS_BUILTIN;
+  attach_args.options[APP_OPTIONS_RX_FIFO_SIZE] = fifo_size;
+  attach_args.options[APP_OPTIONS_TX_FIFO_SIZE] = fifo_size;
+  rv = vnet_application_attach (&attach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_attach %d", rv);
+
+  sm =
+    segment_manager_get (SEGMENT_MANAGER_GET_INDEX_FROM_HANDLE
+			 (attach_args.segment_handle));
+  SEG_MGR_TEST ((sm != 0), "segment_manager_get %p", sm);
+
+  /* initial status : (0 / 2MB) */
+  fs0 = segment_manager_get_segment (sm, 0);
+  rv = fifo_segment_get_mem_status (fs0);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  /* allocate a fifo : 128KB x2 */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo, &tx_fifo);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+
+  svm_fifo_set_size (rx_fifo, size_1MB);
+  svm_fifo_set_size (tx_fifo, size_1MB);
+
+  fs = segment_manager_get_segment (sm, rx_fifo->segment_index);
+  SEG_MGR_TEST ((fs == fs0), "fs %p", fs);
+
+  /* fill fifos (but not add chunks) */
+  svm_fifo_enqueue (rx_fifo, fifo_size - 1, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size - 1, data);
+
+  /* 256KB+ / 2048KB+ => ~12% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* grow fifos */
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+
+  /* 8 chunks : 49% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* grow fifos */
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+
+  /* 10 chunks : 61% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* grow fifos */
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+
+  /* 14 chunks : 85% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_HIGH_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  /* shrink fifos */
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+
+  /* 10 chunks : 61% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  /* grow fifos */
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size, data);
+
+  /* 14 chunks : 85% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_HIGH_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+
+
+  /* 10 chunks : 61% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* shrink fifos */
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+  svm_fifo_dequeue_drop (tx_fifo, fifo_size);
+
+  /* 2 chunks : 12% */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  vnet_app_detach_args_t detach_args = {
+    .app_index = attach_args.app_index,
+    .api_client_index = ~0,
+  };
+  rv = vnet_application_detach (&detach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_detach %d", rv);
+
+  return 0;
+}
+
+static int
+segment_manager_test_pressure_2 (vlib_main_t * vm, unformat_input_t * input)
+{
+  int rv, i;
+  segment_manager_t *sm;
+  fifo_segment_t *fs0, *fs;
+  svm_fifo_t *rx_fifo, *tx_fifo;
+  uword app_seg_size = size_2MB;
+  u32 fifo_size = size_4KB;
+  u64 options[APP_OPTIONS_N_OPTIONS];
+  u8 data[size_4KB];
+
+  memset (&options, 0, sizeof (options));
+
+  vnet_app_attach_args_t attach_args = {
+    .api_client_index = ~0,
+    .options = options,
+    .namespace_id = 0,
+    .session_cb_vft = &dummy_session_cbs,
+    .name = format (0, "segment_manager_test_pressure_1"),
+  };
+
+  attach_args.options[APP_OPTIONS_SEGMENT_SIZE] = app_seg_size;
+  attach_args.options[APP_OPTIONS_FLAGS] = APP_OPTIONS_FLAGS_IS_BUILTIN;
+  attach_args.options[APP_OPTIONS_RX_FIFO_SIZE] = fifo_size;
+  attach_args.options[APP_OPTIONS_TX_FIFO_SIZE] = fifo_size;
+  rv = vnet_application_attach (&attach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_attach %d", rv);
+
+  sm =
+    segment_manager_get (SEGMENT_MANAGER_GET_INDEX_FROM_HANDLE
+			 (attach_args.segment_handle));
+  SEG_MGR_TEST ((sm != 0), "segment_manager_get %p", sm);
+
+  /* initial status : (0 / 2MB) */
+  fs0 = segment_manager_get_segment (sm, 0);
+  fifo_segment_update_free_bytes (fs0);
+  rv = fifo_segment_get_mem_status (fs0);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  /* allocate fifos : 4KB x2 */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo, &tx_fifo);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+
+  svm_fifo_set_size (rx_fifo, size_2MB);
+  svm_fifo_set_size (tx_fifo, size_2MB);
+
+  /* fill fifos (but not add chunks) */
+  svm_fifo_enqueue (rx_fifo, fifo_size - 1, data);
+  svm_fifo_enqueue (tx_fifo, fifo_size - 1, data);
+
+  fs = segment_manager_get_segment (sm, rx_fifo->segment_index);
+
+  /* grow fifos */
+  for (i = 0; i < 509; ++i)
+    {
+      svm_fifo_enqueue (rx_fifo, fifo_size, data);
+    }
+
+  /* 510 chunks : 100% of 2MB */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_HIGH_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* this fifo growth is expected to fail */
+  rv = svm_fifo_enqueue (rx_fifo, fifo_size, data);
+  SEG_MGR_TEST ((rv == SVM_FIFO_EGROW), "svm_fifo_enqueue %d", rv);
+
+  /* then, no-memory is detected */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_MEMORY),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* shrink fifos */
+  for (i = 0; i < 20; ++i)
+    {
+      svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+    }
+
+  /* 489 chunks : 96%, it is high-pressure level
+   * but the reached-mem-limit record is not reset
+   * so the no-memory state lasts.
+   */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_MEMORY),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* shrink fifos */
+  for (i = 0; i < 133; ++i)
+    {
+      svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+    }
+
+  /* 356 chunks : 70% of 2MB */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* shrink fifos */
+  for (i = 0; i < 354; ++i)
+    {
+      svm_fifo_dequeue_drop (rx_fifo, fifo_size);
+    }
+
+  /* 2 chunks : 3% of 2MB */
+  fifo_segment_update_free_bytes (fs);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+
+  vnet_app_detach_args_t detach_args = {
+    .app_index = attach_args.app_index,
+    .api_client_index = ~0,
+  };
+  rv = vnet_application_detach (&detach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_detach %d", rv);
+
+  return 0;
+}
+
+static int
+segment_manager_test_fifo_balanced_alloc (vlib_main_t * vm,
+					  unformat_input_t * input)
+{
+  int rv, i, fs_index;
+  segment_manager_t *sm;
+  fifo_segment_t *fs[4];
+  svm_fifo_t *rx_fifo[4], *tx_fifo[4];
+  uword app_seg_size = size_2MB;
+  u32 fifo_size = size_4KB;
+  u64 options[APP_OPTIONS_N_OPTIONS];
+  u8 data[size_4KB];
+
+  memset (&options, 0, sizeof (options));
+
+  vnet_app_attach_args_t attach_args = {
+    .api_client_index = ~0,
+    .options = options,
+    .namespace_id = 0,
+    .session_cb_vft = &dummy_session_cbs,
+    .name = format (0, "segment_manager_test_pressure_1"),
+  };
+
+  attach_args.options[APP_OPTIONS_SEGMENT_SIZE] = app_seg_size;
+  attach_args.options[APP_OPTIONS_FLAGS] = APP_OPTIONS_FLAGS_IS_BUILTIN;
+  attach_args.options[APP_OPTIONS_RX_FIFO_SIZE] = fifo_size;
+  attach_args.options[APP_OPTIONS_TX_FIFO_SIZE] = fifo_size;
+  rv = vnet_application_attach (&attach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_attach %d", rv);
+
+  sm =
+    segment_manager_get (SEGMENT_MANAGER_GET_INDEX_FROM_HANDLE
+			 (attach_args.segment_handle));
+  SEG_MGR_TEST ((sm != 0), "segment_manager_get %p", sm);
+
+  /* initial status : (0 / 2MB) */
+  fs[0] = segment_manager_get_segment (sm, 0);
+  rv = fifo_segment_get_mem_status (fs[0]);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* allocate fifos : 4KB x2 */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo[0], &tx_fifo[0]);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+  SEG_MGR_TEST ((rx_fifo[0]->segment_index == 0),
+		"segment_index %d", rx_fifo[0]->segment_index);
+  SEG_MGR_TEST ((tx_fifo[0]->segment_index == 0),
+		"segment_index %d", tx_fifo[0]->segment_index);
+
+  /* grow fifos */
+  svm_fifo_set_size (rx_fifo[0], size_1MB);
+  for (i = 0; i < 200; ++i)
+    {
+      svm_fifo_enqueue (rx_fifo[0], fifo_size, data);
+    }
+
+  /* add another 2MB segment */
+  fs_index = segment_manager_add_segment (sm, size_2MB);
+  SEG_MGR_TEST ((fs_index == 1), "fs_index %d", fs_index);
+
+  /* allocate fifos : 4KB x2
+   * expected to be allocated on the newer segment,
+   * because the usage of the first segment is high.
+   */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo[1], &tx_fifo[1]);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+  SEG_MGR_TEST ((rx_fifo[1]->segment_index == 1),
+		"segment_index %d", rx_fifo[1]->segment_index);
+  SEG_MGR_TEST ((tx_fifo[1]->segment_index == 1),
+		"segment_index %d", tx_fifo[1]->segment_index);
+
+  /* allocate fifos : 4KB x2
+   * expected to be allocated on the newer segment.
+   */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo[2], &tx_fifo[2]);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+  SEG_MGR_TEST ((rx_fifo[2]->segment_index == 1),
+		"segment_index %d", rx_fifo[2]->segment_index);
+  SEG_MGR_TEST ((tx_fifo[2]->segment_index == 1),
+		"segment_index %d", tx_fifo[2]->segment_index);
+
+  /* grow fifos, so the usage of the secong segment becomes
+   * higher than the first one.
+   */
+  svm_fifo_set_size (rx_fifo[1], size_1MB);
+  for (i = 0; i < 400; ++i)
+    {
+      svm_fifo_enqueue (rx_fifo[1], fifo_size, data);
+    }
+
+  /* allocate fifos : 4KB x2
+   * expected to be allocated on the first segment.
+   */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo[3], &tx_fifo[3]);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+  SEG_MGR_TEST ((rx_fifo[3]->segment_index == 0),
+		"segment_index %d", rx_fifo[3]->segment_index);
+  SEG_MGR_TEST ((tx_fifo[3]->segment_index == 0),
+		"segment_index %d", tx_fifo[3]->segment_index);
+
+
+
+  vnet_app_detach_args_t detach_args = {
+    .app_index = attach_args.app_index,
+    .api_client_index = ~0,
+  };
+  rv = vnet_application_detach (&detach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_detach %d", rv);
+
+  return 0;
+}
+
+static int
+segment_manager_test_fifo_ops (vlib_main_t * vm, unformat_input_t * input)
+{
+  int rv, i;
+  segment_manager_t *sm;
+  fifo_segment_t *fs;
+  svm_fifo_t *rx_fifo, *tx_fifo;
+  uword app_seg_size = size_2MB, most_grown = 0;
+  u32 fifo_size = size_4KB;
+  u32 max_dequeue = 0;
+  u64 options[APP_OPTIONS_N_OPTIONS];
+  u8 data[size_128KB];
+
+  memset (&options, 0, sizeof (options));
+
+  vnet_app_attach_args_t attach_args = {
+    .api_client_index = ~0,
+    .options = options,
+    .namespace_id = 0,
+    .session_cb_vft = &dummy_session_cbs,
+    .name = format (0, "segment_manager_test_pressure_1"),
+  };
+
+  attach_args.options[APP_OPTIONS_SEGMENT_SIZE] = app_seg_size;
+  attach_args.options[APP_OPTIONS_FLAGS] = APP_OPTIONS_FLAGS_IS_BUILTIN;
+  attach_args.options[APP_OPTIONS_RX_FIFO_SIZE] = fifo_size;
+  attach_args.options[APP_OPTIONS_TX_FIFO_SIZE] = fifo_size;
+  rv = vnet_application_attach (&attach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_attach %d", rv);
+
+  sm =
+    segment_manager_get (SEGMENT_MANAGER_GET_INDEX_FROM_HANDLE
+			 (attach_args.segment_handle));
+  SEG_MGR_TEST ((sm != 0), "segment_manager_get %p", sm);
+
+  /* initial status : (0 / 2MB) */
+  fs = segment_manager_get_segment (sm, 0);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* allocate fifos : 4KB x2 */
+  rv = segment_manager_alloc_session_fifos (sm,
+					    vlib_get_thread_index (),
+					    &rx_fifo, &tx_fifo);
+  SEG_MGR_TEST ((rv == 0), "segment_manager_alloc_session_fifos %d", rv);
+
+  /* check the initial fifo size : 4KB */
+  rv = svm_fifo_size (rx_fifo);
+  SEG_MGR_TEST ((rv == size_4KB), "svm_fifo_size %d", rv);
+
+  /* fill 4KB */
+  rv = svm_fifo_enqueue (rx_fifo, size_4KB, data);
+  SEG_MGR_TEST ((rv == size_4KB), "svm_fifo_enqueue %d", rv);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == size_4KB), "max_dequeue %u", max_dequeue);
+
+  /* grow the fifo size : 4KB -> 8KB */
+  svm_fifo_set_size (rx_fifo, size_8KB);
+  rv = svm_fifo_size (rx_fifo);
+  SEG_MGR_TEST ((rv == size_8KB), "svm_fifo_size %d", rv);
+
+  /* verify that fifo cannot grow larger than the fifo size */
+  /* 4KB + 8KB > 8KB, so only 4KB is queued */
+  rv = svm_fifo_enqueue (rx_fifo, size_8KB, data);
+  SEG_MGR_TEST ((rv == size_4KB), "svm_fifo_enqueue %d", rv);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == size_8KB), "max_dequeue %u", max_dequeue);
+
+  /* grow the fifo size : 8KB -> 16KB */
+  svm_fifo_set_size (rx_fifo, size_16KB);
+
+  /* 8KB + 4KB = 12KB */
+  svm_fifo_enqueue (rx_fifo, size_4KB, data);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == size_12KB), "max_dequeue %u", max_dequeue);
+
+  /* grow the fifo size : 16KB -> 32KB */
+  svm_fifo_set_size (rx_fifo, size_32KB);
+
+  /* 12KB + 8KB = 20KB */
+  svm_fifo_enqueue (rx_fifo, size_8KB, data);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == size_20KB), "max_dequeue %u", max_dequeue);
+
+  /* grow the fifo size : 32KB -> 64KB */
+  svm_fifo_set_size (rx_fifo, size_64KB);
+
+  /* 20KB + 32KB = 52KB */
+  svm_fifo_enqueue (rx_fifo, size_32KB, data);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == size_52KB), "max_dequeue %u", max_dequeue);
+
+  /* bulk enqueue */
+  for (i = 0; i < 55; ++i)
+    {
+      svm_fifo_set_size (rx_fifo, svm_fifo_size (rx_fifo) + size_32KB);
+      svm_fifo_enqueue (rx_fifo, size_32KB, data);
+    }
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == (size_52KB + size_32KB * 55)),
+		"max_dequeue %u", max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_HIGH_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+  most_grown = svm_fifo_size (rx_fifo);
+
+  /* dequeue */
+  svm_fifo_dequeue_drop (rx_fifo, size_20KB);
+  svm_fifo_dequeue_drop (rx_fifo, size_32KB);
+
+  /* bulk dequeue */
+  for (i = 0; i < 20; ++i)
+    svm_fifo_dequeue_drop (rx_fifo, size_32KB);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == (size_32KB * 35)), "max_dequeue %u",
+		max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* bulk dequeue */
+  for (i = 0; i < 10; ++i)
+    svm_fifo_dequeue_drop (rx_fifo, size_32KB);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == (size_32KB * 25)), "max_dequeue %u",
+		max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* bulk enqueue */
+  for (i = 0; i < 30; ++i)
+    svm_fifo_enqueue (rx_fifo, size_32KB, data);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == (size_32KB * 55)), "max_dequeue %u",
+		max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_HIGH_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* bulk dequeue */
+  for (i = 0; i < 20; ++i)
+    svm_fifo_dequeue_drop (rx_fifo, size_32KB);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == (size_32KB * 35)), "max_dequeue %u",
+		max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_LOW_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* bulk dequeue */
+  for (i = 0; i < 35; ++i)
+    svm_fifo_dequeue_drop (rx_fifo, size_32KB);
+  max_dequeue = svm_fifo_max_dequeue (rx_fifo);
+  SEG_MGR_TEST ((max_dequeue == 0), "max_dequeue %u", max_dequeue);
+  rv = fifo_segment_get_mem_status (fs);
+  SEG_MGR_TEST ((rv == MEMORY_PRESSURE_NO_PRESSURE),
+		"fifo_segment_get_mem_status %s", states_str[rv]);
+
+  /* (virtual) fifo size is still large as it is not updated */
+  SEG_MGR_TEST ((rx_fifo->size == most_grown), "rx_fifo->size %u",
+		rx_fifo->size);
+
+  vnet_app_detach_args_t detach_args = {
+    .app_index = attach_args.app_index,
+    .api_client_index = ~0,
+  };
+  rv = vnet_application_detach (&detach_args);
+  SEG_MGR_TEST ((rv == 0), "vnet_application_detach %d", rv);
+
+  return 0;
+}
+
+
+static clib_error_t *
+segment_manager_test (vlib_main_t * vm,
+		      unformat_input_t * input, vlib_cli_command_t * cmd_arg)
+{
+  int res = 0;
+
+  vnet_session_enable_disable (vm, 1);
+
+  while (unformat_check_input (input) != UNFORMAT_END_OF_INPUT)
+    {
+      if (unformat (input, "pressure_levels_1"))
+	res = segment_manager_test_pressure_1 (vm, input);
+      else if (unformat (input, "pressure_levels_2"))
+	res = segment_manager_test_pressure_2 (vm, input);
+      else if (unformat (input, "alloc"))
+	res = segment_manager_test_fifo_balanced_alloc (vm, input);
+      else if (unformat (input, "fifo_ops"))
+	res = segment_manager_test_fifo_ops (vm, input);
+
+      else if (unformat (input, "all"))
+	{
+	  if ((res = segment_manager_test_pressure_1 (vm, input)))
+	    goto done;
+	  if ((res = segment_manager_test_pressure_2 (vm, input)))
+	    goto done;
+	  if ((res = segment_manager_test_fifo_balanced_alloc (vm, input)))
+	    goto done;
+	  if ((res = segment_manager_test_fifo_ops (vm, input)))
+	    goto done;
+	}
+      else
+	break;
+    }
+
+done:
+  if (res)
+    return clib_error_return (0, "Segment manager unit test failed.");
+  return 0;
+}
+
+/* *INDENT-OFF* */
+VLIB_CLI_COMMAND (tcp_test_command, static) =
+{
+  .path = "test segment-manager",
+  .short_help = "test segment manager [pressure_levels_1]"
+                "[pressure_level_2][alloc][fifo_ops][all]",
+  .function = segment_manager_test,
+};
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */
+
diff --git a/src/svm/fifo_segment.c b/src/svm/fifo_segment.c
index 49cd1161a4b..88600b84e36 100644
--- a/src/svm/fifo_segment.c
+++ b/src/svm/fifo_segment.c
@@ -15,6 +15,12 @@
 
 #include <svm/fifo_segment.h>
 
+static char *fifo_segment_mem_status_strings[] = {
+#define _(sym,str) str,
+  foreach_segment_mem_status
+#undef _
+};
+
 /**
  * Fifo segment free space
  *
@@ -53,6 +59,26 @@ fsh_update_free_bytes (fifo_segment_header_t * fsh)
   clib_atomic_store_rel_n (&fsh->n_free_bytes, fsh_free_space (fsh));
 }
 
+static inline void
+fsh_cached_bytes_add (fifo_segment_header_t * fsh, int size)
+{
+  clib_atomic_fetch_add_rel (&fsh->n_cached_bytes, size);
+}
+
+static inline void
+fsh_cached_bytes_sub (fifo_segment_header_t * fsh, int size)
+{
+  clib_atomic_fetch_sub_rel (&fsh->n_cached_bytes, size);
+}
+
+static inline uword
+fsh_n_cached_bytes (fifo_segment_header_t * fsh)
+{
+  uword n_cached = clib_atomic_load_relax_n (&fsh->n_cached_bytes);
+  ASSERT (n_cached >= 0);
+  return n_cached;
+}
+
 static void
 fsh_check_mem (fifo_segment_header_t * fsh)
 {
@@ -133,6 +159,7 @@ fifo_segment_init (fifo_segment_t * fs)
   ssvm_pop_heap (oldheap);
 
   fsh->n_free_bytes = fsh_free_space (fsh);
+  fsh->n_cached_bytes = 0;
   max_chunks = fsh->n_free_bytes / FIFO_SEGMENT_MIN_FIFO_SIZE;
   fsh->n_reserved_bytes = (max_chunks / 4) * sizeof (rb_node_t);
   sh->ready = 1;
@@ -349,6 +376,7 @@ fs_try_alloc_fifo_freelist_multi_chunk (fifo_segment_header_t * fsh,
 		  c->next = fss->free_chunks[fl_index];
 		  fss->free_chunks[fl_index] = c;
 		  fss->n_fl_chunk_bytes += fl_size;
+		  n_alloc -= fl_size;
 		  data_bytes += fl_size;
 		}
 	      first = last = 0;
@@ -372,6 +400,7 @@ fs_try_alloc_fifo_freelist_multi_chunk (fifo_segment_header_t * fsh,
   f->start_chunk = first;
   f->end_chunk = last;
   fss->n_fl_chunk_bytes -= n_alloc;
+  fsh_cached_bytes_sub (fsh, n_alloc);
   return f;
 }
 
@@ -420,6 +449,7 @@ fs_try_alloc_fifo_batch (fifo_segment_header_t * fsh,
     }
 
   fss->n_fl_chunk_bytes += batch_size * rounded_data_size;
+  fsh_cached_bytes_add (fsh, batch_size * rounded_data_size);
   fsh_free_bytes_sub (fsh, size);
 
   return 0;
@@ -450,7 +480,10 @@ fs_try_alloc_fifo (fifo_segment_header_t * fsh, fifo_segment_slice_t * fss,
     {
       f = fs_try_alloc_fifo_freelist (fss, fl_index, data_bytes);
       if (f)
-	goto done;
+	{
+	  fsh_cached_bytes_sub (fsh, fs_freelist_index_to_size (fl_index));
+	  goto done;
+	}
     }
 
   fsh_check_mem (fsh);
@@ -462,6 +495,8 @@ fs_try_alloc_fifo (fifo_segment_header_t * fsh, fifo_segment_slice_t * fss,
 	goto done;
 
       f = fs_try_alloc_fifo_freelist (fss, fl_index, data_bytes);
+      if (f)
+	fsh_cached_bytes_sub (fsh, fs_freelist_index_to_size (fl_index));
       goto done;
     }
   if (fifo_sz <= n_free_bytes)
@@ -520,6 +555,7 @@ fsh_alloc_chunk (fifo_segment_header_t * fsh, u32 slice_index, u32 chunk_size)
       fss->free_chunks[fl_index] = c->next;
       c->next = 0;
       fss->n_fl_chunk_bytes -= fs_freelist_index_to_size (fl_index);
+      fsh_cached_bytes_sub (fsh, fs_freelist_index_to_size (fl_index));
     }
 
 done:
@@ -530,10 +566,12 @@ done:
 }
 
 static void
-fsh_slice_collect_chunks (fifo_segment_slice_t * fss, svm_fifo_chunk_t * cur)
+fsh_slice_collect_chunks (fifo_segment_header_t * fsh,
+			  fifo_segment_slice_t * fss, svm_fifo_chunk_t * cur)
 {
   svm_fifo_chunk_t *next;
   int fl_index;
+  u32 n_collect = 0;
 
   clib_spinlock_lock (&fss->chunk_lock);
 
@@ -545,10 +583,13 @@ fsh_slice_collect_chunks (fifo_segment_slice_t * fss, svm_fifo_chunk_t * cur)
       cur->enq_rb_index = RBTREE_TNIL_INDEX;
       cur->deq_rb_index = RBTREE_TNIL_INDEX;
       fss->free_chunks[fl_index] = cur;
-      fss->n_fl_chunk_bytes += fs_freelist_index_to_size (fl_index);
+      n_collect += fs_freelist_index_to_size (fl_index);
       cur = next;
     }
 
+  fss->n_fl_chunk_bytes += n_collect;
+  fsh_cached_bytes_add (fsh, n_collect);
+
   clib_spinlock_unlock (&fss->chunk_lock);
 }
 
@@ -558,7 +599,7 @@ fsh_collect_chunks (fifo_segment_header_t * fsh, u32 slice_index,
 {
   fifo_segment_slice_t *fss;
   fss = fsh_slice_get (fsh, slice_index);
-  fsh_slice_collect_chunks (fss, cur);
+  fsh_slice_collect_chunks (fsh, fss, cur);
 }
 
 /**
@@ -644,7 +685,7 @@ fifo_segment_free_fifo (fifo_segment_t * fs, svm_fifo_t * f)
   fss->free_fifos = f;
 
   /* Free fifo chunks */
-  fsh_slice_collect_chunks (fss, f->start_chunk);
+  fsh_slice_collect_chunks (fsh, fss, f->start_chunk);
 
   f->start_chunk = f->end_chunk = 0;
   f->head_chunk = f->tail_chunk = f->ooo_enq = f->ooo_deq = 0;
@@ -746,6 +787,7 @@ fifo_segment_prealloc_fifo_chunks (fifo_segment_t * fs, u32 slice_index,
       c->next = fss->free_chunks[fl_index];
       fss->free_chunks[fl_index] = c;
       cmem += sizeof (*c) + rounded_data_size;
+      fsh_cached_bytes_add (fsh, rounded_data_size);
     }
 
   fss->n_fl_chunk_bytes += batch_size * rounded_data_size;
@@ -926,12 +968,36 @@ fifo_segment_update_free_bytes (fifo_segment_t * fs)
 }
 
 uword
+fifo_segment_size (fifo_segment_t * fs)
+{
+  return fs->ssvm.ssvm_size;
+}
+
+u8
+fsh_has_reached_mem_limit (fifo_segment_header_t * fsh)
+{
+  return (fsh->flags & FIFO_SEGMENT_F_MEM_LIMIT) ? 1 : 0;
+}
+
+void
+fsh_reset_mem_limit (fifo_segment_header_t * fsh)
+{
+  fsh->flags &= ~FIFO_SEGMENT_F_MEM_LIMIT;
+}
+
+uword
 fifo_segment_free_bytes (fifo_segment_t * fs)
 {
   return fsh_n_free_bytes (fs->h);
 }
 
 uword
+fifo_segment_cached_bytes (fifo_segment_t * fs)
+{
+  return fsh_n_cached_bytes (fs->h);
+}
+
+uword
 fifo_segment_fl_chunk_bytes (fifo_segment_t * fs)
 {
   fifo_segment_header_t *fsh = fs->h;
@@ -974,6 +1040,52 @@ fifo_segment_get_slice_fifo_list (fifo_segment_t * fs, u32 slice_index)
   return fss->fifos;
 }
 
+u8
+fifo_segment_get_mem_usage (fifo_segment_t * fs)
+{
+  uword size, in_use;
+
+  size = fifo_segment_size (fs);
+  in_use =
+    size - fifo_segment_free_bytes (fs) - fifo_segment_cached_bytes (fs);
+  return (in_use * 100) / size;
+}
+
+fifo_segment_mem_status_t
+fifo_segment_determine_status (fifo_segment_header_t * fsh, u8 usage)
+{
+  if (!fsh->high_watermark || !fsh->low_watermark)
+    return MEMORY_PRESSURE_NO_PRESSURE;
+
+  /* once the no-memory is detected, the status continues
+   * until memory usage gets below the high watermark
+   */
+  if (fsh_has_reached_mem_limit (fsh))
+    {
+      if (usage >= fsh->high_watermark)
+	return MEMORY_PRESSURE_NO_MEMORY;
+      else
+	fsh_reset_mem_limit (fsh);
+    }
+
+  if (usage >= fsh->high_watermark)
+    return MEMORY_PRESSURE_HIGH_PRESSURE;
+
+  else if (usage >= fsh->low_watermark)
+    return MEMORY_PRESSURE_LOW_PRESSURE;
+
+  return MEMORY_PRESSURE_NO_PRESSURE;
+}
+
+fifo_segment_mem_status_t
+fifo_segment_get_mem_status (fifo_segment_t * fs)
+{
+  fifo_segment_header_t *fsh = fs->h;
+  u8 usage = fifo_segment_get_mem_usage (fs);
+
+  return fifo_segment_determine_status (fsh, usage);
+}
+
 u8 *
 format_fifo_segment_type (u8 * s, va_list * args)
 {
@@ -1003,6 +1115,7 @@ format_fifo_segment (u8 * s, va_list * args)
   int verbose __attribute__ ((unused)) = va_arg (*args, int);
   uword est_chunk_bytes, est_free_seg_bytes, free_chunks;
   uword chunk_bytes = 0, free_seg_bytes, chunk_size;
+  uword tracked_cached_bytes;
   fifo_segment_header_t *fsh;
   fifo_segment_slice_t *fss;
   svm_fifo_chunk_t *c;
@@ -1010,6 +1123,9 @@ format_fifo_segment (u8 * s, va_list * args)
   char *address;
   size_t size;
   int i;
+  uword allocated, in_use;
+  f64 usage;
+  fifo_segment_mem_status_t mem_st;
 
   indent = format_get_indent (s) + 2;
 
@@ -1068,19 +1184,31 @@ format_fifo_segment (u8 * s, va_list * args)
   est_free_seg_bytes = fifo_segment_free_bytes (fs);
   fifo_segment_update_free_bytes (fs);
   free_seg_bytes = fifo_segment_free_bytes (fs);
+  tracked_cached_bytes = fifo_segment_cached_bytes (fs);
+  allocated = fifo_segment_size (fs);
+  in_use = fifo_segment_size (fs) - est_free_seg_bytes - tracked_cached_bytes;
+  usage = (100.0 * in_use) / allocated;
+  mem_st = fifo_segment_get_mem_status (fs);
 
   s = format (s, "\n%Useg free bytes: %U (%lu) estimated: %U (%lu)\n",
 	      format_white_space, indent + 2, format_memory_size,
 	      free_seg_bytes, free_seg_bytes, format_memory_size,
 	      est_free_seg_bytes, est_free_seg_bytes);
-  s = format (s, "%Uchunk free bytes: %U (%lu) estimated: %U (%lu)\n",
-	      format_white_space, indent + 2, format_memory_size, chunk_bytes,
-	      chunk_bytes, format_memory_size, est_chunk_bytes,
-	      est_chunk_bytes);
-  s = format (s, "%Ufifo hdr free bytes: %U (%u) reserved %U (%lu)\n",
-	      format_white_space, indent + 2, format_memory_size, fifo_hdr,
-	      fifo_hdr, format_memory_size, fsh->n_reserved_bytes,
-	      fsh->n_reserved_bytes);
+  s =
+    format (s,
+	    "%Uchunk free bytes: %U (%lu) estimated: %U (%lu) tracked: %U (%lu)\n",
+	    format_white_space, indent + 2, format_memory_size, chunk_bytes,
+	    chunk_bytes, format_memory_size, est_chunk_bytes, est_chunk_bytes,
+	    format_memory_size, tracked_cached_bytes, tracked_cached_bytes);
+  s =
+    format (s, "%Ufifo hdr free bytes: %U (%u) reserved %U (%lu)\n",
+	    format_white_space, indent + 2, format_memory_size, fifo_hdr,
+	    fifo_hdr, format_memory_size, fsh->n_reserved_bytes,
+	    fsh->n_reserved_bytes);
+  s =
+    format (s, "%Usegment usage: %.2f%% (%U / %U) %s\n", format_white_space,
+	    indent + 2, usage, format_memory_size, in_use, format_memory_size,
+	    allocated, fifo_segment_mem_status_strings[mem_st]);
   s = format (s, "\n");
 
   return s;
diff --git a/src/svm/fifo_segment.h b/src/svm/fifo_segment.h
index 85548063972..4e950f6b4f0 100644
--- a/src/svm/fifo_segment.h
+++ b/src/svm/fifo_segment.h
@@ -39,6 +39,30 @@ typedef enum fifo_segment_flags_
   FIFO_SEGMENT_F_MEM_LIMIT = 1 << 2,
 } fifo_segment_flags_t;
 
+#define foreach_segment_mem_status	\
+_(NO_PRESSURE, "No pressure")		\
+_(LOW_PRESSURE, "Low pressure")		\
+_(HIGH_PRESSURE, "High pressure")	\
+_(NO_MEMORY, "No memory")
+
+typedef enum
+{
+#define _(sym,str)  MEMORY_PRESSURE_##sym,
+  foreach_segment_mem_status
+#undef _
+    MEMORY_N_PRESSURE,
+} fifo_segment_mem_status_t;
+
+#if 0
+typedef enum fifo_segment_mem_status_
+{
+  MEMORY_PRESSURE_NO_PRESSURE,
+  MEMORY_PRESSURE_LOW_PRESSURE,
+  MEMORY_PRESSURE_HIGH_PRESSURE,
+  MEMORY_PRESSURE_NO_MEMORY,
+} fifo_segment_mem_status_t;
+#endif
+
 typedef struct
 {
   ssvm_private_t ssvm;		/**< ssvm segment data */
@@ -147,6 +171,31 @@ void fsh_collect_chunks (fifo_segment_header_t * fsh, u32 slice_index,
 			 svm_fifo_chunk_t * cur);
 
 /**
+ * Fifo segment has reached mem limit
+ *
+ * @param fsh           fifo segment header
+ * @return              1 (if reached) or 0 (otherwise)
+ */
+u8 fsh_has_reached_mem_limit (fifo_segment_header_t * fsh);
+
+/**
+ * Fifo segment reset mem limit flag
+ *
+ * @param fs            fifo segment
+ */
+void fsh_reset_mem_limit (fifo_segment_header_t * fsh);
+
+/**
+ * Fifo segment allocated size
+ *
+ * Returns fifo segment's allocated size
+ *
+ * @param fs            fifo segment
+ * @return              allocated size in bytes
+ */
+uword fifo_segment_size (fifo_segment_t * fs);
+
+/**
  * Fifo segment estimate of number of free bytes
  *
  * Returns fifo segment's internal estimate of the number of free bytes.
@@ -169,6 +218,16 @@ uword fifo_segment_free_bytes (fifo_segment_t * fs);
 void fifo_segment_update_free_bytes (fifo_segment_t * fs);
 
 /**
+ * Fifo segment number of cached bytes
+ *
+ * Returns fifo segment's number of cached bytes.
+ *
+ * @param fs            fifo segment
+ * @return              cached bytes
+ */
+uword fifo_segment_cached_bytes (fifo_segment_t * fs);
+
+/**
  * Number of bytes on chunk free lists
  *
  * @param fs		fifo segment
@@ -189,6 +248,11 @@ u32 fifo_segment_num_free_fifos (fifo_segment_t * fs);
  */
 u32 fifo_segment_num_free_chunks (fifo_segment_t * fs, u32 size);
 
+u8 fifo_segment_get_mem_usage (fifo_segment_t * fs);
+fifo_segment_mem_status_t fifo_segment_determine_status
+  (fifo_segment_header_t * fsh, u8 usage);
+fifo_segment_mem_status_t fifo_segment_get_mem_status (fifo_segment_t * fs);
+
 void fifo_segment_main_init (fifo_segment_main_t * sm, u64 baseva,
 			     u32 timeout_in_seconds);
 
diff --git a/src/svm/fifo_types.h b/src/svm/fifo_types.h
index 3e6a14eea7d..f0a286d46a0 100644
--- a/src/svm/fifo_types.h
+++ b/src/svm/fifo_types.h
@@ -112,11 +112,14 @@ struct fifo_segment_header_
   fifo_segment_slice_t *slices;		/** Fixed array of slices */
   ssvm_shared_header_t *ssvm_sh;	/**< Pointer to fs ssvm shared hdr */
   uword n_free_bytes;			/**< Segment free bytes */
+  uword n_cached_bytes;			/**< Cached bytes */
   u32 n_active_fifos;			/**< Number of active fifos */
   u32 n_reserved_bytes;			/**< Bytes not to be allocated */
   u32 max_log2_chunk_size;		/**< Max log2(chunk size) for fs */
   u8 flags;				/**< Segment flags */
   u8 n_slices;				/**< Number of slices */
+  u8 high_watermark;			/**< Memory pressure watermark high */
+  u8 low_watermark;			/**< Memory pressure watermark low */
 };
 
 #endif /* SRC_SVM_FIFO_TYPES_H_ */
diff --git a/src/vnet/session/application.c b/src/vnet/session/application.c
index 646d6b6de2c..0dcb3e6ad94 100644
--- a/src/vnet/session/application.c
+++ b/src/vnet/session/application.c
@@ -550,6 +550,10 @@ application_alloc_and_init (app_init_args_t * a)
     props->use_mq_eventfd = 1;
   if (options[APP_OPTIONS_TLS_ENGINE])
     app->tls_engine = options[APP_OPTIONS_TLS_ENGINE];
+  if (options[APP_OPTIONS_HIGH_WATERMARK])
+    props->high_watermark = options[APP_OPTIONS_HIGH_WATERMARK];
+  if (options[APP_OPTIONS_LOW_WATERMARK])
+    props->low_watermark = options[APP_OPTIONS_LOW_WATERMARK];
   props->segment_type = seg_type;
 
   /* Add app to lookup by api_client_index table */
@@ -698,8 +702,7 @@ application_alloc_worker_and_init (application_t * app, app_worker_t ** wrk)
   sm = segment_manager_alloc ();
   sm->app_wrk_index = app_wrk->wrk_index;
 
-  if ((rv = segment_manager_init (sm, app->sm_properties.segment_size,
-				  app->sm_properties.prealloc_fifos)))
+  if ((rv = segment_manager_init (sm)))
     {
       app_worker_free (app_wrk);
       return rv;
diff --git a/src/vnet/session/application_interface.h b/src/vnet/session/application_interface.h
index 552fec1b6ef..d03994fc1f9 100644
--- a/src/vnet/session/application_interface.h
+++ b/src/vnet/session/application_interface.h
@@ -201,6 +201,8 @@ typedef enum
   APP_OPTIONS_PROXY_TRANSPORT,
   APP_OPTIONS_ACCEPT_COOKIE,
   APP_OPTIONS_TLS_ENGINE,
+  APP_OPTIONS_HIGH_WATERMARK,
+  APP_OPTIONS_LOW_WATERMARK,
   APP_OPTIONS_N_OPTIONS
 } app_attach_options_index_t;
 
diff --git a/src/vnet/session/segment_manager.c b/src/vnet/session/segment_manager.c
index 0a54b96a125..25fbd6f2c04 100644
--- a/src/vnet/session/segment_manager.c
+++ b/src/vnet/session/segment_manager.c
@@ -29,6 +29,8 @@ typedef struct segment_manager_main_
   u32 default_fifo_size;		/**< default rx/tx fifo size */
   u32 default_segment_size;		/**< default fifo segment size */
   u32 default_app_mq_size;		/**< default app msg q size */
+  u8 default_high_watermark;		/**< default high watermark % */
+  u8 default_low_watermark;		/**< default low watermark % */
 } segment_manager_main_t;
 
 static segment_manager_main_t sm_main;
@@ -54,6 +56,8 @@ segment_manager_props_init (segment_manager_props_t * props)
   props->rx_fifo_size = sm_main.default_fifo_size;
   props->tx_fifo_size = sm_main.default_fifo_size;
   props->evt_q_size = sm_main.default_app_mq_size;
+  props->high_watermark = sm_main.default_high_watermark;
+  props->low_watermark = sm_main.default_low_watermark;
   props->n_slices = vlib_num_workers () + 1;
   return props;
 }
@@ -160,6 +164,13 @@ segment_manager_add_segment (segment_manager_t * sm, uword segment_size)
    */
   fs_index = fs - sm->segments;
 
+  /*
+   * Set watermarks in segment
+   */
+  fs->h->high_watermark = sm->high_watermark;
+  fs->h->low_watermark = sm->low_watermark;
+  fs->h->flags &= ~FIFO_SEGMENT_F_MEM_LIMIT;
+
 done:
 
   if (vlib_num_workers ())
@@ -307,11 +318,12 @@ segment_manager_alloc (void)
  * Returns error if ssvm segment(s) allocation fails.
  */
 int
-segment_manager_init (segment_manager_t * sm, uword first_seg_size,
-		      u32 prealloc_fifo_pairs)
+segment_manager_init (segment_manager_t * sm)
 {
   u32 rx_fifo_size, tx_fifo_size, pair_size;
   u32 rx_rounded_data_size, tx_rounded_data_size;
+  uword first_seg_size;
+  u32 prealloc_fifo_pairs;
   u64 approx_total_size, max_seg_size = ((u64) 1 << 32) - (128 << 10);
   segment_manager_props_t *props;
   fifo_segment_t *segment;
@@ -319,7 +331,13 @@ segment_manager_init (segment_manager_t * sm, uword first_seg_size,
   int seg_index, i;
 
   props = segment_manager_properties_get (sm);
-  first_seg_size = clib_max (first_seg_size, sm_main.default_segment_size);
+  first_seg_size = clib_max (props->segment_size,
+			     sm_main.default_segment_size);
+  prealloc_fifo_pairs = props->prealloc_fifos;
+
+  segment_manager_set_watermarks (sm,
+				  props->high_watermark,
+				  props->low_watermark);
 
   if (prealloc_fifo_pairs)
     {
@@ -434,6 +452,27 @@ segment_manager_get_if_valid (u32 index)
   return pool_elt_at_index (sm_main.segment_managers, index);
 }
 
+static fifo_segment_t *
+find_max_free_segment (segment_manager_t * sm, u32 thread_index)
+{
+  fifo_segment_t *cur, *fs = 0;
+  uword free_bytes, max_free_bytes = 0;
+
+  clib_rwlock_reader_lock (&sm->segments_rwlock);
+  /* *INDENT-OFF* */
+  pool_foreach (cur, sm->segments, ({
+    if ((free_bytes = fifo_segment_free_bytes (cur)) > max_free_bytes)
+      {
+        max_free_bytes = free_bytes;
+        fs = cur;
+      }
+  }));
+  /* *INDENT-ON* */
+  clib_rwlock_reader_unlock (&sm->segments_rwlock);
+
+  return fs;
+}
+
 u32
 segment_manager_index (segment_manager_t * sm)
 {
@@ -569,19 +608,25 @@ segment_manager_alloc_session_fifos (segment_manager_t * sm,
   /*
    * Find the first free segment to allocate the fifos in
    */
+  fs = find_max_free_segment (sm, thread_index);
 
-  /* *INDENT-OFF* */
-  segment_manager_foreach_segment_w_lock (fs, sm, ({
-    alloc_fail = segment_manager_try_alloc_fifos (fs,
-                                                  thread_index,
-                                                  props->rx_fifo_size,
-                                                  props->tx_fifo_size,
-                                                  rx_fifo, tx_fifo);
-    /* Exit with lock held, drop it after notifying app */
-    if (!alloc_fail)
-      goto alloc_success;
-  }));
-  /* *INDENT-ON* */
+  if (fs)
+    {
+      clib_rwlock_reader_lock (&sm->segments_rwlock);
+      alloc_fail = segment_manager_try_alloc_fifos (fs, thread_index,
+						    props->rx_fifo_size,
+						    props->tx_fifo_size,
+						    rx_fifo, tx_fifo);
+      /* On success, keep lock until fifos are initialized */
+      if (!alloc_fail)
+	goto alloc_success;
+
+      segment_manager_segment_reader_unlock (sm);
+    }
+  else
+    {
+      alloc_fail = 1;
+    }
 
 alloc_check:
 
@@ -787,6 +832,8 @@ segment_manager_main_init (segment_manager_main_init_args_t * a)
   sm->default_fifo_size = 1 << 12;
   sm->default_segment_size = 1 << 20;
   sm->default_app_mq_size = 128;
+  sm->default_high_watermark = 80;
+  sm->default_low_watermark = 50;
 }
 
 static clib_error_t *
@@ -910,6 +957,18 @@ segment_manager_format_sessions (segment_manager_t * sm, int verbose)
   clib_rwlock_reader_unlock (&sm->segments_rwlock);
 }
 
+void
+segment_manager_set_watermarks (segment_manager_t * sm,
+				u8 high_watermark, u8 low_watermark)
+{
+  ASSERT (high_watermark >= 0 && high_watermark <= 100 &&
+	  low_watermark >= 0 && low_watermark <= 100 &&
+	  low_watermark <= high_watermark);
+
+  sm->high_watermark = high_watermark;
+  sm->low_watermark = low_watermark;
+}
+
 /*
  * fd.io coding-style-patch-verification: ON
  *
diff --git a/src/vnet/session/segment_manager.h b/src/vnet/session/segment_manager.h
index 52f89eef171..cad95509b2e 100644
--- a/src/vnet/session/segment_manager.h
+++ b/src/vnet/session/segment_manager.h
@@ -34,6 +34,8 @@ typedef struct _segment_manager_props
   u8 n_slices;				/**< number of fs slices/threads */
   ssvm_segment_type_t segment_type;	/**< seg type: if set to SSVM_N_TYPES,
 					     private segments are used */
+  u8 high_watermark;			/**< memory usage high watermark % */
+  u8 low_watermark;			/**< memory usage low watermark % */
 } segment_manager_props_t;
 
 typedef struct _segment_manager
@@ -58,6 +60,9 @@ typedef struct _segment_manager
    * App event queue allocated in first segment
    */
   svm_msg_q_t *event_queue;
+
+  u8 high_watermark;
+  u8 low_watermark;
 } segment_manager_t;
 
 typedef struct segment_manager_main_init_args_
@@ -69,8 +74,7 @@ typedef struct segment_manager_main_init_args_
 #define SEGMENT_MANAGER_INVALID_APP_INDEX ((u32) ~0)
 
 segment_manager_t *segment_manager_alloc (void);
-int segment_manager_init (segment_manager_t * sm, uword first_seg_size,
-			  u32 prealloc_fifo_pairs);
+int segment_manager_init (segment_manager_t * sm);
 
 /**
  * Cleanup segment manager
@@ -118,6 +122,9 @@ int segment_manager_try_alloc_fifos (fifo_segment_t * fs,
 void segment_manager_dealloc_fifos (svm_fifo_t * rx_fifo,
 				    svm_fifo_t * tx_fifo);
 
+void segment_manager_set_watermarks (segment_manager_t * sm,
+				     u8 high_watermark, u8 low_watermark);
+
 u8 segment_manager_has_fifos (segment_manager_t * sm);
 
 svm_msg_q_t *segment_manager_alloc_queue (fifo_segment_t * fs,