Re: [PATCH] 9pfs: log warning if msize <= 8192

[Greg Kurz <groug@kaod.org>]

On Wed, 02 Sep 2020 14:52:33 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Mittwoch, 2. September 2020 14:25:47 CEST Daniel P. Berrangé wrote:
> > On Wed, Sep 02, 2020 at 01:22:49PM +0200, Christian Schoenebeck wrote:
> > > It is essential to choose a reasonable high value for 'msize' to avoid
> > > severe degraded file I/O performance. This parameter has to be chosen
> > > on client/guest side, and a Linux client defaults to an 'msize' of only
> > > 8192 if the user did not explicitly specify a value for 'msize'.
> > > 
> > > Unfortunately many users are not aware that they should specify an
> > > appropriate value for 'msize' to avoid severe performance issues, so
> > > log a performance warning on host side in that case to make it more
> > > clear.
> > 
> > What is a more reasonable "msize" value to pick instead of 8k ?
> > ie at what msize is I/O not several degraded ?
> 
> A good value depends on the file I/O potential of the underlying storage on 
> host side, and then you still would need to trade off between performance 
> profit and additional RAM costs, i.e. with growing msize (RAM occupation), 
> performance still increases, but performance delta will shrink continuously.
> 
> So in practice you might e.g. choose anything between 10MiB ... >100MiB for a 
> SATA spindle disk storage, and a much higher value for anything PCIe based 
> flash storage. So a user probably should benchmark and decide what's 
> reasonable for the intended use case.
> 
> > If there a reason that Linux can't pick a better default ?
> 
> I was not involved when that default value was picked on Linux side, so I 
> don't know why exactly this value (8192) had been chosen as default 'msize' 
> years ago.
> 

The original size back in 2005 was 9000:

[greg@bahia kernel-linus]$ git show 9e82cf6a802a7 | grep 9000
+	v9ses->maxdata = 9000;
+	if (v9ses->maxdata != 9000)

which was later converted to 8192.

I couldn't find any hint on why such a small size was chosen.

Maybe you can try to contact 9pfs father ?

Eric Van Hensbergen <ericvh@gmail.com>

> It certainly (sh/c)ould be higher, as it is already close to the theoreticaly 
> minimum msize of 4096. However how should a Linux guest automatically pick a 
> reasonable msize if it does not have any knowlege of host's storage features?
> 
> But even if this will be addressed on Linux kernel side, I still think users 
> of old kernels should be made aware of this issue, as it is not obvious to the 
> user.
> 

I tend to agree. Until linux decides of a better default, we should only
warn the user if they decide to go below the current one.

Cheers,

--
Greg

> Best regards,
> Christian Schoenebeck
> 
>