[pulseaudio-discuss] [PATCH] iochannel: Avoid unnecessary wakeup after successful write

[David Henningsson]

To save some CPU (in low latency scenarios), don't re-enable the
"writable" event after it has succeeded. It is very likely the next
write will succeed right away too.

This means that we always need to handle EAGAIN/EWOULDBLOCK as a
successful write of 0 bytes, so I also verified that all callers to
pa_iochannel_write handled this correctly.

Signed-off-by: David Henningsson <david.henningsson at canonical.com>
---
 src/modules/module-esound-sink.c |    2 +-
 src/pulsecore/iochannel.c        |   17 ++++++++++++++---
 src/pulsecore/ioline.c           |    5 +----
 src/pulsecore/protocol-esound.c  |    4 ----
 src/pulsecore/protocol-http.c    |    4 ----
 src/pulsecore/protocol-simple.c  |    4 ----
 6 files changed, 16 insertions(+), 20 deletions(-)

diff --git a/src/modules/module-esound-sink.c b/src/modules/module-esound-sink.c
index 40ef5bc..9cb7a71 100644
--- a/src/modules/module-esound-sink.c
+++ b/src/modules/module-esound-sink.c
@@ -345,7 +345,7 @@ static int do_write(struct userdata *u) {
     if (u->write_data) {
         pa_assert(u->write_index < u->write_length);
 
-        if ((r = pa_iochannel_write(u->io, (uint8_t*) u->write_data + u->write_index, u->write_length - u->write_index)) <= 0) {
+        if ((r = pa_iochannel_write(u->io, (uint8_t*) u->write_data + u->write_index, u->write_length - u->write_index)) < 0) {
             pa_log("write() failed: %s", pa_cstrerror(errno));
             return -1;
         }
diff --git a/src/pulsecore/iochannel.c b/src/pulsecore/iochannel.c
index fa3d767..ed8be23 100644
--- a/src/pulsecore/iochannel.c
+++ b/src/pulsecore/iochannel.c
@@ -223,11 +223,22 @@ ssize_t pa_iochannel_write(pa_iochannel*io, const void*data, size_t l) {
     pa_assert(l);
     pa_assert(io->ofd >= 0);
 
-    if ((r = pa_write(io->ofd, data, l, &io->ofd_type)) >= 0) {
-        io->writable = io->hungup = FALSE;
-        enable_events(io);
+    r = pa_write(io->ofd, data, l, &io->ofd_type);
+
+    if ((size_t) r == l)
+        return r; /* Fast path - we almost always successfully write everything */
+
+    if (r < 0) {
+        if (errno == EINTR || errno == EAGAIN || errno == EWOULDBLOCK)
+            r = 0;
+        else
+            return r;
     }
 
+    /* Partial write - let's get a notification when we can write more */
+    io->writable = io->hungup = FALSE;
+    enable_events(io);
+
     return r;
 }
 
diff --git a/src/pulsecore/ioline.c b/src/pulsecore/ioline.c
index a18188d..e4d2604 100644
--- a/src/pulsecore/ioline.c
+++ b/src/pulsecore/ioline.c
@@ -351,10 +351,7 @@ static int do_write(pa_ioline *l) {
 
     while (l->io && !l->dead && pa_iochannel_is_writable(l->io) && l->wbuf_valid_length > 0) {
 
-        if ((r = pa_iochannel_write(l->io, l->wbuf+l->wbuf_index, l->wbuf_valid_length)) <= 0) {
-
-            if (r < 0 && errno == EAGAIN)
-                break;
+        if ((r = pa_iochannel_write(l->io, l->wbuf+l->wbuf_index, l->wbuf_valid_length)) < 0) {
 
             if (r < 0 && errno != EPIPE)
                 pa_log("write(): %s", pa_cstrerror(errno));
diff --git a/src/pulsecore/protocol-esound.c b/src/pulsecore/protocol-esound.c
index 52aa2c5..ff4afc6 100644
--- a/src/pulsecore/protocol-esound.c
+++ b/src/pulsecore/protocol-esound.c
@@ -1226,10 +1226,6 @@ static int do_write(connection *c) {
         pa_memblock_unref(chunk.memblock);
 
         if (r < 0) {
-
-            if (r < 0 && (errno == EINTR || errno == EAGAIN))
-                return 0;
-
             pa_log("write(): %s", pa_cstrerror(errno));
             return -1;
         }
diff --git a/src/pulsecore/protocol-http.c b/src/pulsecore/protocol-http.c
index 400150f..52571d3 100644
--- a/src/pulsecore/protocol-http.c
+++ b/src/pulsecore/protocol-http.c
@@ -162,10 +162,6 @@ static int do_write(struct connection *c) {
     pa_memblock_unref(chunk.memblock);
 
     if (r < 0) {
-
-        if (errno == EINTR || errno == EAGAIN)
-            return 0;
-
         pa_log("write(): %s", pa_cstrerror(errno));
         return -1;
     }
diff --git a/src/pulsecore/protocol-simple.c b/src/pulsecore/protocol-simple.c
index 9242e68..0e7c6e0 100644
--- a/src/pulsecore/protocol-simple.c
+++ b/src/pulsecore/protocol-simple.c
@@ -229,10 +229,6 @@ static int do_write(connection *c) {
     pa_memblock_unref(chunk.memblock);
 
     if (r < 0) {
-
-        if (errno == EINTR || errno == EAGAIN)
-            return 0;
-
         pa_log("write(): %s", pa_cstrerror(errno));
         return -1;
     }
-- 
1.7.9.5