[libvirt RFC v2] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[root]

From: Claudio Fontana <cfontana at suse.de>

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 at 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