Re: [RFC PATCH 0/5] 9p: Performance improvements for build workloads

[Remi Pommarel <repk@triplefau.lt>]

Hi Dominique,

On Sun, Sep 14, 2025 at 09:34:11PM +0900, Dominique Martinet wrote:
> Remi Pommarel wrote on Sun, Aug 31, 2025 at 09:03:38PM +0200:
> > This patchset introduces several performance optimizations for the 9p
> > filesystem when used with cache=loose option (exclusive or read only
> > mounts). These improvements particularly target workloads with frequent
> > lookups of non-existent paths and repeated symlink resolutions.
> 
> Sorry for slow reply, I think a negative cache and symlink cache make
> sense.
> I haven't tested these yet, and there's a conversion to the "new" mount
> API that's brewing and will conflict with 2nd patch, but I'll be happy
> to take these patches as time allows.
> What was the reason this was sent as RFC, does something require more work?
> 
> I can't comment on io_wait_event_killable, it makes sense to me as well
> but it's probably more appropriate to send through the scheduler tree.
> 

RFC was mainly here to know if a io_wait_event_killable() would made
sense before getting the scheduler tree involved. Also as it is my first
contribution in v9fs (and fs subsystem) wanted to be sure I wasn't
missing something obvious, caching could be a complex subject to grasp.
This also comes with some drawbacks, if for example server removes a
shared file or modify a symlink the client will be desynchronized, so I
wanted first to be sure we were ok with that when using cache=loose.

I'll try to monitor the new mount API and rebase the series when that
get merged. I'll probably separate the io_wait_event_killable() in its
own patchset though.

> 
> > The third patch extends page cache usage to symlinks by allowing
> > p9_client_readlink() results to be cached. Resolving symlink is
> > apparently something done quite frequently during the build process and
> > avoiding the cost of a 9P RPC call round trip for already known symlinks
> > helps reduce the build time to 1m26.602s, outperforming the virtiofs
> > setup.
> 
> That's rather impressive!
> (I assume virtiofs does not have such negative lookup or symlink cache so
> they'll catch up soon enough if someone cares? But that's no reason to
> refuse this with cache=loose)
> 

virtiofs does have negative lookup (when used with cache=always) and
symlink caches (this serie is even quite a bit inspired by what fuse
does). I don't really know what makes virtiofs a bit slower here, I
haven't dig into it either though but won't be surprised it could easily
catch up.

> > Further investigation may be needed to address the remaining gap with
> > native build performance. Using the last two patches it appears there is
> > still a fair amount of time spent waiting for I/O, though. This could be
> > related to the two systematic RPC calls made when opening a file (one to
> > clone the fid and another one to open the file). Maybe reusing fids or
> > openned files could potentially reduce client/server transactions and
> > bring performance even closer to native levels ? But that are just
> > random thoughs I haven't dig enough yet.
> 
> Another thing I tried ages ago was making clunk asynchronous,
> but that didn't go well;
> protocol-wise clunk errors are ignored so I figured it was safe enough
> to just fire it in the background, but it caused some regressions I
> never had time to look into...
> 
> As for reusing fids, I'm not sure it's obvious because of things like
> locking that basically consider one open file = one fid;
> I think we're already re-using fids when we can, but I guess it's
> technically possible to mark a fid as shared and only clone it if an
> operation that requires an exclusive fid is done...?
> I'm not sure I want to go down that hole though, sounds like an easy way
> to mess up and give someone access to data they shouldn't be able to
> access by sharing a fid opened by another user or something more
> subtle..

Yes I gave that a bit more thinking and came up with quite the same
conclusion, I then gave up on this idea. The asynchronous clunk seems
interesting though, maybe I'll take a look into that.

Thanks for your time.

-- 
Remi