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[Qemu-devel] [PATCH 3/4] Improve character device read performance
From: |
Anthony Liguori |
Subject: |
[Qemu-devel] [PATCH 3/4] Improve character device read performance |
Date: |
Sun, 16 Jul 2006 13:14:29 -0500 |
User-agent: |
Thunderbird 1.5.0.4 (X11/20060615) |
Regards,
Anthony Liguori
# HG changeset patch
# User address@hidden
# Node ID 32d5f77cf5e8c5a15b38b1fa86f588f374ebd3ad
# Parent 0e52f303222037247747558d43d72b5b89817828
Sometimes partial reads occur even though there's still data to be read. This
patch makes sure that we read all the data that's available to read.
This has a huge performance impact for the shmem character device since.
diff -r 0e52f3032220 -r 32d5f77cf5e8 vl.c
--- a/vl.c Sun Jul 16 17:04:01 2006
+++ b/vl.c Sun Jul 16 17:23:21 2006
@@ -1205,6 +1205,29 @@
return s->max_size;
}
+static int fd_read_blocks(int fd)
+{
+ fd_set rfds;
+ struct timeval tv;
+ int ret;
+
+ again:
+ FD_ZERO(&rfds);
+ FD_SET(fd, &rfds);
+
+ tv.tv_sec = 0;
+ tv.tv_usec = 0;
+
+ ret = select(fd + 1, &rfds, NULL, NULL, &tv);
+ if (ret == -1) {
+ if (errno == EINTR)
+ goto again;
+ return -1;
+ }
+
+ return !FD_ISSET(fd, &rfds);
+}
+
static void fd_chr_read(void *opaque)
{
CharDriverState *chr = opaque;
@@ -1217,7 +1240,25 @@
len = s->max_size;
if (len == 0)
return;
- size = read(s->fd_in, buf, len);
+
+ for (size = 0; size < len;) {
+ int l;
+ l = read(s->fd_in, buf + size, len - size);
+ if (l == -1) {
+ if (errno == EINTR)
+ continue;
+ break;
+ } else if (l == 0) {
+ break;
+ }
+
+ size += l;
+
+ if (size < len && fd_read_blocks(s->fd_in))
+ break;
+ }
+
+
if (size > 0) {
s->fd_read(s->fd_opaque, buf, size);
}
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