Re: [PATCH v2 0/3] 9p: Performance improvements for build workloads

[Dominique Martinet <asmadeus@codewreck.org>]

Remi Pommarel wrote on Wed, Jan 21, 2026 at 08:56:07PM +0100:
> 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.
> 
> The very state of the art benchmark consisting of cloning a fresh
> hostap repository and building hostapd and wpa_supplicant for hwsim
> tests (cd tests/hwsim; time ./build.sh) in a VM running on a 9pfs rootfs
> (with trans=virtio,cache=loose options) has been used to test those
> optimizations impact.
> 
> For reference, the build takes 0m56.492s on my laptop natively while it
> completes in 2m18.702sec on the VM. This represents a significant
> performance penalty considering running the same build on a VM using a
> virtiofs rootfs (with "--cache always" virtiofsd option) takes around
> 1m32.141s. This patchset aims to bring the 9pfs build time close to
> that of virtiofs, rather than the native host time, as a realistic
> expectation.
> 
> This first two patches in this series focus on keeping negative dentries
> in the cache, ensuring that subsequent lookups for paths known to not
> exist do not require redundant 9P RPC calls. This optimization reduces
> the time needed for the compiler to search for header files across known
> locations. These two patches introduce a new mount option, ndentrytmo,
> which specifies the number of ms to keep the dentry in the cache. Using
> ndentrytmo=-1 (keeping the negative dentry indifinetly) shrunk build
> time to 1m46.198s.
> 
> 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.
> 
> Here is summary of the different hostapd/wpa_supplicant build times:
> 
>   - Baseline (no patch): 2m18.702s
>   - negative dentry caching (patches 1-2): 1m46.198s (23% improvement)
>   - Above + symlink caching (patches 1-3): 1m26.302s (an additional 18%
>     improvement, 37% in total)
> 
> With this ~37% performance gain, 9pfs with cache=loose can compete with
> virtiofs for (at least) this specific scenario. Although this benchmark
> is not the most typical, I do think that these caching optimizations
> could benefit a wide range of other workflows as well.

Thank you!

We've had a couple of regressions lately so I'll take a week or two to
run some proper tests first, but overall looks good to me, I just wanted
to acknowledge the patches early.
(as such it likely won't make 6.20 but should hopefully go into the next
one)
-- 
Dominique