svm_fifo rework to avoid contention on cursize

Problems Addressed:
- Contention of cursize by producer and consumer.
- Reduce the no of modulo operations.

Changes:
- Synchronization between producer and consumer changed from cursize
  to head and tail indexes
  Implications: reduces the usable size of fifo by 1.
- Using weaker memory ordering C++11 atomics to access head and tail
  based on producer and consumer role.
- Head and tail indexes are unsigned 32 bit integers. Additions and
  subtraction on them are implicit 32 bit Modulo operation.
- Adding weaker memory ordering variants of max_enq, max_deq, is_empty
  and is_full Using them appropriately in all places.

Perfomance improvement (iperf3 via Hoststack):

iperf3 Server: Marvell ThunderX2(AArch64) - iperf3 Client: Skylake(x86)
   ~6%(256 rxd/txd) - ~11%(2048 rxd/txd)

Change-Id: I1d484e000e437430fdd5a819657d1c6b62443018
Signed-off-by: Sirshak Das <sirshak.das@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/vnet/session-apps/http_server.c b/src/vnet/session-apps/http_server.c
index 4547a4dc4ef..c7e06bfbb2d 100644
--- a/src/vnet/session-apps/http_server.c
+++ b/src/vnet/session-apps/http_server.c
@@ -503,7 +503,7 @@ session_rx_request (http_session_t * hs)
   int n_read;
 
   cursize = vec_len (hs->rx_buf);
-  max_dequeue = svm_fifo_max_dequeue (hs->rx_fifo);
+  max_dequeue = svm_fifo_max_dequeue_cons (hs->rx_fifo);
   if (PREDICT_FALSE (max_dequeue == 0))
     return -1;
 
@@ -511,7 +511,7 @@ session_rx_request (http_session_t * hs)
   n_read = app_recv_stream_raw (hs->rx_fifo, hs->rx_buf + cursize,
 				max_dequeue, 0, 0 /* peek */ );
   ASSERT (n_read == max_dequeue);
-  if (svm_fifo_is_empty (hs->rx_fifo))
+  if (svm_fifo_is_empty_cons (hs->rx_fifo))
     svm_fifo_unset_event (hs->rx_fifo);
 
   _vec_len (hs->rx_buf) = cursize + n_read;