[libvirt RFCv4] virfile: set pipe size in virFileWrapperFdNew to improve throughput
currently the only user of virFileWrapperFdNew is the qemu driver;
virsh save is very slow with a default pipe size.
This change improves throughput by ~400% on fast nvme or ramdisk.
Best value currently measured is 1MB, which happens to be also
the kernel default for the pipe-max-size.
Signed-off-by: Claudio Fontana <cfontana(a)suse.de>
---
src/util/virfile.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 46 insertions(+)
see v2 at
https://listman.redhat.com/archives/libvir-list/2022-March/229423.html
Changes v3 -> v4:
* changed INFO and WARN messages to DEBUG (Daniel)
Changes v2 -> v3:
* removed reading of max-pipe-size from procfs,
instead make multiple attempts on EPERM with smaller sizes.
In the regular case, this should succeed on the first try.
(Daniel)
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..87539be0b9 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -201,6 +201,50 @@ struct _virFileWrapperFd {
};
#ifndef WIN32
+
+#ifdef __linux__
+
+/**
+ * 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. A value of 1MB has been measured to improve virsh save
+ * by 400% in ideal conditions. We retry multiple times with smaller sizes
+ * on EPERM to account for possible small values of /proc/sys/fs/pipe-max-size.
+ *
+ * OS note: only for linux, on other OS this is a no-op.
+ */
+static void
+virFileWrapperSetPipeSize(int fd)
+{
+ int sz;
+
+ for (sz = 1024 * 1024; sz >= 64 * 1024; sz /= 2) {
+ int rv = fcntl(fd, F_SETPIPE_SZ, sz);
+ if (rv < 0 && errno == EPERM) {
+ VIR_DEBUG("EPERM trying to set fd %d pipe size to %d", fd, sz);
+ continue; /* retry with half the size */
+ }
+ if (rv < 0) {
+ break;
+ }
+ VIR_DEBUG("fd %d pipe size adjusted to %d", fd, sz);
+ return;
+ }
+ virReportSystemError(errno, "%s", _("unable to set pipe size, data
transfer might be slow"));
+}
+
+#else /* !__linux__ */
+static void virFileWrapperSetPipeSize(int fd)
+{
+ return;
+}
+#endif /* !__linux__ */
+
+
/**
* virFileWrapperFdNew:
* @fd: pointer to fd to wrap
@@ -282,6 +326,8 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
+ virFileWrapperSetPipeSize(pipefd[!output]);
+
if (output) {
virCommandSetInputFD(ret->cmd, pipefd[0]);
virCommandSetOutputFD(ret->cmd, fd);
--
2.35.1