Re: Can not set high msize with virtio-9p (Was: Re: virtiofs vs 9p performance)

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Freitag, 19. Februar 2021 17:08:48 CET Vivek Goyal wrote:
> On Fri, Sep 25, 2020 at 10:06:41AM +0200, Christian Schoenebeck wrote:
> > On Freitag, 25. September 2020 00:10:23 CEST Vivek Goyal wrote:
> > > In my testing, with cache=none, virtiofs performed better than 9p in
> > > all the fio jobs I was running. For the case of cache=auto  for virtiofs
> > > (with xattr enabled), 9p performed better in certain write workloads. I
> > > have identified root cause of that problem and working on
> > > HANDLE_KILLPRIV_V2 patches to improve WRITE performance of virtiofs
> > > with cache=auto and xattr enabled.
> > 
> > Please note, when it comes to performance aspects, you should set a
> > reasonable high value for 'msize' on 9p client side:
> > https://wiki.qemu.org/Documentation/9psetup#msize
> 
> Hi Christian,
> 
> I am not able to set msize to a higher value. If I try to specify msize
> 16MB, and then read back msize from /proc/mounts, it sees to cap it
> at 512000. Is that intended?

9p server side in QEMU does not perform any msize capping. The code in this
case is very simple, it's just what you see in function v9fs_version():

https://github.com/qemu/qemu/blob/6de76c5f324904c93e69f9a1e8e4fd0bd6f6b57a/hw/9pfs/9p.c#L1332

> $ mount -t 9p -o trans=virtio,version=9p2000.L,cache=none,msize=16777216
> hostShared /mnt/virtio-9p
> 
> $ cat /proc/mounts | grep 9p
> hostShared /mnt/virtio-9p 9p
> rw,sync,dirsync,relatime,access=client,msize=512000,trans=virtio 0 0
> 
> I am using 5.11 kernel.

Must be something on client (guest kernel) side. I don't see this here with
guest kernel 4.9.0 happening with my setup in a quick test:

$ cat /etc/mtab | grep 9p
svnRoot / 9p rw,dirsync,relatime,trans=virtio,version=9p2000.L,msize=104857600,cache=mmap 0 0
$ 

Looks like the root cause of your issue is this:

struct p9_client *p9_client_create(const char *dev_name, char *options)
{
	...
	if (clnt->msize > clnt->trans_mod->maxsize)
		clnt->msize = clnt->trans_mod->maxsize;

https://github.com/torvalds/linux/blob/f40ddce88593482919761f74910f42f4b84c004b/net/9p/client.c#L1045

Best regards,
Christian Schoenebeck