[pulseaudio-discuss] [PATCH 1/2] pstream: Optimise write of smaller packages

[David Henningsson]

During a stream, most packets sent are either memblocks (with SHM info),
or requests for more data. These are only slightly bigger than the
header.

This patch makes it possible to write these packages in one write
instead of two: a memcpy of just a few bytes is worth saving extra
syscalls for write and poll.

Signed-off-by: David Henningsson <david.henningsson at canonical.com>
---
 src/pulsecore/pstream.c |   35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/src/pulsecore/pstream.c b/src/pulsecore/pstream.c
index 7538a86..d92178f 100644
--- a/src/pulsecore/pstream.c
+++ b/src/pulsecore/pstream.c
@@ -74,6 +74,8 @@ typedef uint32_t pa_pstream_descriptor[PA_PSTREAM_DESCRIPTOR_MAX];
 
 #define PA_PSTREAM_DESCRIPTOR_SIZE (PA_PSTREAM_DESCRIPTOR_MAX*sizeof(uint32_t))
 
+#define MINIBUF_SIZE (256)
+
 /* To allow uploading a single sample in one frame, this value should be the
  * same size (16 MB) as PA_SCACHE_ENTRY_SIZE_MAX from pulsecore/core-scache.h.
  */
@@ -118,11 +120,14 @@ struct pa_pstream {
     pa_bool_t dead;
 
     struct {
-        pa_pstream_descriptor descriptor;
+        union {
+            uint8_t minibuf[MINIBUF_SIZE];
+            pa_pstream_descriptor descriptor;
+        };
         struct item_info* current;
-        uint32_t shm_info[PA_PSTREAM_SHM_MAX];
         void *data;
         size_t index;
+        int minibuf_validsize;
         pa_memchunk memchunk;
     } write;
 
@@ -469,9 +474,9 @@ static void prepare_next_write_item(pa_pstream *p) {
 
     if (!p->write.current)
         return;
-
     p->write.index = 0;
     p->write.data = NULL;
+    p->write.minibuf_validsize = 0;
     pa_memchunk_reset(&p->write.memchunk);
 
     p->write.descriptor[PA_PSTREAM_DESCRIPTOR_LENGTH] = 0;
@@ -486,6 +491,11 @@ static void prepare_next_write_item(pa_pstream *p) {
         p->write.data = p->write.current->packet->data;
         p->write.descriptor[PA_PSTREAM_DESCRIPTOR_LENGTH] = htonl((uint32_t) p->write.current->packet->length);
 
+        if (p->write.current->packet->length <= MINIBUF_SIZE - PA_PSTREAM_DESCRIPTOR_SIZE) {
+            memcpy(&p->write.minibuf[PA_PSTREAM_DESCRIPTOR_SIZE], p->write.data, p->write.current->packet->length);
+            p->write.minibuf_validsize = PA_PSTREAM_DESCRIPTOR_SIZE + p->write.current->packet->length;
+        }
+
     } else if (p->write.current->type == PA_PSTREAM_ITEM_SHMRELEASE) {
 
         p->write.descriptor[PA_PSTREAM_DESCRIPTOR_FLAGS] = htonl(PA_FLAG_SHMRELEASE);
@@ -512,6 +522,8 @@ static void prepare_next_write_item(pa_pstream *p) {
         if (p->use_shm) {
             uint32_t block_id, shm_id;
             size_t offset, length;
+            uint32_t *shm_info = (uint32_t *) &p->write.minibuf[PA_PSTREAM_DESCRIPTOR_SIZE];
+            size_t shm_size = sizeof(uint32_t) * PA_PSTREAM_SHM_MAX;
 
             pa_assert(p->export);
 
@@ -525,13 +537,13 @@ static void prepare_next_write_item(pa_pstream *p) {
                 flags |= PA_FLAG_SHMDATA;
                 send_payload = FALSE;
 
-                p->write.shm_info[PA_PSTREAM_SHM_BLOCKID] = htonl(block_id);
-                p->write.shm_info[PA_PSTREAM_SHM_SHMID] = htonl(shm_id);
-                p->write.shm_info[PA_PSTREAM_SHM_INDEX] = htonl((uint32_t) (offset + p->write.current->chunk.index));
-                p->write.shm_info[PA_PSTREAM_SHM_LENGTH] = htonl((uint32_t) p->write.current->chunk.length);
+                shm_info[PA_PSTREAM_SHM_BLOCKID] = htonl(block_id);
+                shm_info[PA_PSTREAM_SHM_SHMID] = htonl(shm_id);
+                shm_info[PA_PSTREAM_SHM_INDEX] = htonl((uint32_t) (offset + p->write.current->chunk.index));
+                shm_info[PA_PSTREAM_SHM_LENGTH] = htonl((uint32_t) p->write.current->chunk.length);
 
-                p->write.descriptor[PA_PSTREAM_DESCRIPTOR_LENGTH] = htonl(sizeof(p->write.shm_info));
-                p->write.data = p->write.shm_info;
+                p->write.descriptor[PA_PSTREAM_DESCRIPTOR_LENGTH] = htonl(shm_size);
+                p->write.minibuf_validsize = PA_PSTREAM_DESCRIPTOR_SIZE + shm_size;
             }
 /*             else */
 /*                 pa_log_warn("Failed to export memory block."); */
@@ -568,7 +580,10 @@ static int do_write(pa_pstream *p) {
     if (!p->write.current)
         return 0;
 
-    if (p->write.index < PA_PSTREAM_DESCRIPTOR_SIZE) {
+    if (p->write.minibuf_validsize > 0) {
+        d = p->write.minibuf + p->write.index;
+        l = p->write.minibuf_validsize - p->write.index;
+    } else if (p->write.index < PA_PSTREAM_DESCRIPTOR_SIZE) {
         d = (uint8_t*) p->write.descriptor + p->write.index;
         l = PA_PSTREAM_DESCRIPTOR_SIZE - p->write.index;
     } else {
-- 
1.7.9.5