[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

[Daniel P. Berrangé]

On Thu, Mar 17, 2022 at 11:12:11AM +0100, Claudio Fontana wrote:
> On 3/16/22 1:17 PM, Claudio Fontana wrote:
> > On 3/14/22 6:48 PM, Daniel P. Berrangé wrote:
> >> On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana wrote:
> >>> On 3/14/22 6:17 PM, Daniel P. Berrangé wrote:
> >>>> On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio Fontana wrote:
> >>>>> the first user is the qemu driver,
> >>>>>
> >>>>> virsh save/resume would slow to a crawl with a default pipe size (64k).
> >>>>>
> >>>>> This improves the situation by 400%.
> >>>>>
> >>>>> Going through io_helper still seems to incur in some penalty (~15%-ish)
> >>>>> compared with direct qemu migration to a nc socket to a file.
> >>>>>
> >>>>> Signed-off-by: Claudio Fontana <cfontana at suse.de>
> >>>>> ---
> >>>>>  src/qemu/qemu_driver.c    |  6 +++---
> >>>>>  src/qemu/qemu_saveimage.c | 11 ++++++-----
> >>>>>  src/util/virfile.c        | 12 ++++++++++++
> >>>>>  src/util/virfile.h        |  1 +
> >>>>>  4 files changed, 22 insertions(+), 8 deletions(-)
> >>>>>
> >>>>> Hello, I initially thought this to be a qemu performance issue,
> >>>>> so you can find the discussion about this in qemu-devel:
> >>>>>
> >>>>> "Re: bad virsh save /dev/null performance (600 MiB/s max)"
> >>>>>
> >>>>> https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html


> Current results show these experimental averages maximum throughput
> migrating to /dev/null per each FdWrapper Pipe Size (as per QEMU QMP
> "query-migrate", tests repeated 5 times for each).
> VM Size is 60G, most of the memory effectively touched before migration,
> through user application allocating and touching all memory with
> pseudorandom data.
> 
> 64K:     5200 Mbps (current situation)
> 128K:    5800 Mbps
> 256K:   20900 Mbps
> 512K:   21600 Mbps
> 1M:     22800 Mbps
> 2M:     22800 Mbps
> 4M:     22400 Mbps
> 8M:     22500 Mbps
> 16M:    22800 Mbps
> 32M:    22900 Mbps
> 64M:    22900 Mbps
> 128M:   22800 Mbps
> 
> This above is the throughput out of patched libvirt with multiple Pipe Sizes for the FDWrapper.

Ok, its bouncing around with noise after 1 MB. So I'd suggest that
libvirt attempt to raise the pipe limit to 1 MB by default, but
not try to go higher.

> As for the theoretical limit for the libvirt architecture,
> I ran a qemu migration directly issuing the appropriate QMP
> commands, setting the same migration parameters as per libvirt,
> and then migrating to a socket netcatted to /dev/null via
> {"execute": "migrate", "arguments": { "uri", "unix:///tmp/netcat.sock" } } :
> 
> QMP:    37000 Mbps

> So although the Pipe size improves things (in particular the
> large jump is for the 256K size, although 1M seems a very good value),
> there is still a second bottleneck in there somewhere that
> accounts for a loss of ~14200 Mbps in throughput.

In the above tests with libvirt, were you using the
--bypass-cache flag or not ?

Hopefully use of O_DIRECT doesn't make a difference for
/dev/null, since the I/O is being immediately thrown
away and so ought to never go into I/O cache. 

In terms of the comparison, we still have libvirt iohelper
giving QEMU a pipe, while your test above gives QEMU a
UNIX socket.

So I still wonder if the delta is caused by the pipe vs socket
difference, as opposed to netcat vs libvirt iohelper code.

With regards,
Daniel
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