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

[Christian Schoenebeck]

Hi Eric,

do you remember any specific reason why the default 'msize' for the Linux 
kernel's 9P client was chosen such low as 8 kiB? (see QEMU discussion below).

Best regards,
Christian Schoenebeck

On Mittwoch, 2. September 2020 15:39:34 CEST Greg Kurz wrote:
> 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