[libvirt RFC v2] virfile: set pipe size in virFileWrapperFdNew to improve throughput
virsh save is very slow with a default pipe size, so set a larger one.
This change improves throughput by ~400% on fast nvme or ramdisk,
for the current only user of virFileWrapperFdNew: the qemu driver.
Best value currently measured is 1MB, which happens to be also
the kernel default for the pipe-max-size.
We do not try to use a pipe buffer larger than what the setting
of /proc/sys/fs/pipe-max-size currently allows.
Signed-off-by: Claudio Fontana <cfontana(a)suse.de>
---
src/util/virfile.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 69 insertions(+)
see v1 at
https://listman.redhat.com/archives/libvir-list/2022-March/229252.html
Changes v1 -> v2:
* removed VIR_FILE_WRAPPER_BIG_PIPE, made the new pipe resizing
unconditional (Michal)
* moved code to separate functions (Michal)
* removed ternary op, disliked in libvirt (Michal)
* added #ifdef __linux__ (Ani Sinha)
* try smallest value between currently best measured value (1MB)
and the pipe-max-size setting. If pipe-max-size cannot be read,
try kernel default max (1MB). (Daniel)
diff --git a/src/util/virfile.c b/src/util/virfile.c
index a04f888e06..13bdd42c68 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -201,6 +201,71 @@ struct _virFileWrapperFd {
};
#ifndef WIN32
+
+#ifdef __linux__
+/**
+ * virFileWrapperGetBestPipeSize:
+ *
+ * get the best pipe size to use with virFileWrapper.
+ *
+ * We first check the maximum we are allowed by the system pipe-max-size,
+ * and then use the minimum between that and our tested best value.
+ * This is because a request beyond pipe-max-size may fail with EPERM.
+ * If we are unable to read pipe-max-size, use the kernel default (1MB).
+ *
+ * Return value is the pipe size to use.
+ */
+
+static int virFileWrapperGetBestPipeSize(void)
+{
+ const char path[] = "/proc/sys/fs/pipe-max-size";
+ int best_sz = 1024 * 1024; /* good virsh save results with this size */
+ int max_sz;
+
+ if (virFileReadValueInt(&max_sz, path) < 0) {
+ max_sz = 1024 * 1024; /* this is the kernel default pipe-max-size */
+ VIR_WARN("failed to read %s, trying default %d", path, max_sz);
+ } else if (max_sz > best_sz) {
+ max_sz = best_sz;
+ }
+ return max_sz;
+}
+
+/**
+ * virFileWrapperSetPipeSize:
+ * @fd: the fd of the pipe
+ *
+ * Set best pipe size on the passed file descriptor for bulk transfers of data.
+ *
+ * default pipe size (usually 64K) is generally not suited for large transfers
+ * to fast devices. This has been measured to improve virsh save by 400%
+ * in ideal conditions.
+ *
+ * Return value is 0 on success, -1 and errno set on error.
+ * OS note: only for linux, on other OS this is a no-op.
+ */
+static int
+virFileWrapperSetPipeSize(int fd)
+{
+ int pipe_sz = virFileWrapperGetBestPipeSize();
+ int rv = fcntl(fd, F_SETPIPE_SZ, pipe_sz);
+
+ if (rv < 0) {
+ VIR_ERROR(_("failed to set pipe size to %d (errno=%d)"), pipe_sz,
errno);
+ return -1;
+ }
+ VIR_INFO("fd %d pipe size adjusted to %d", fd, rv);
+ return 0;
+}
+
+#else /* !__linux__ */
+static int virFileWrapperSetPipeSize(int fd)
+{
+ return 0;
+}
+#endif /* !__linux__ */
+
+
/**
* virFileWrapperFdNew:
* @fd: pointer to fd to wrap
@@ -282,6 +347,10 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
+ if (virFileWrapperSetPipeSize(pipefd[!output]) < 0) {
+ virReportError(VIR_ERR_SYSTEM_ERROR, "%s", _("unable to set pipe
size, data transfer might be slow"));
+ }
+
if (output) {
virCommandSetInputFD(ret->cmd, pipefd[0]);
virCommandSetOutputFD(ret->cmd, fd);
--
2.35.1