[libvirt RFCv4] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Claudio Fontana]

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 at 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