[PATCH 0/4 RFC] nfsd: allocate/free session-based DRC slots on demand

NeilBrown
2024-11-13T05:53:57Z

This patch set aims to allocate session-based DRC slots on demand, and
free them when not in use, or when memory is tight.

I've tested with NFSD_MAX_UNUSED_SLOTS set to 1 so that freeing is
overly agreesive, and with lots of printks, and it seems to do the right
thing, though memory pressure has never freed anything - I think you
need several clients with a non-trivial number of slots allocated before
the thresholds in the shrinker code will trigger any freeing.

I haven't made use of the CB_RECALL_SLOT callback. I'm not sure how
useful that is. There are certainly cases where simply setting the
target in a SEQUENCE reply might not be enough, but I doubt they are
very common. You would need a session to be completely idle, with the
last request received on it indicating that lots of slots were still in
use.

Currently we allocate slots one at a time when the last available slot
was used by the client, and only if a NOWAIT allocation can succeed. It
is possible that this isn't quite agreesive enough. When performing a
lot of writeback it can be useful to have lots of slots, but memory
pressure is also likely to build up on the server so GFP_NOWAIT is likely
to fail. Maybe occasionally using a firmer request (outside the
spinlock) would be justified.

We free slots when the number of unused slots passes some threshold -
currently 6 (because ... why not). Possible a hysteresis should be
added so we don't free unused slots for a least N seconds.

When the shrinker wants to apply presure we remove slots equally from
all sessions. Maybe there should be some proportionality but that would
be more complex and I'm not sure it would gain much. Slot 0 can never
be freed of course.

I'm very interested to see what people think of the over-all approach,
and of the specifics of the code.

Thanks,
NeilBrown

[PATCH 1/4] nfsd: remove artificial limits on the session-based DRC
[PATCH 2/4] nfsd: allocate new session-based DRC slots on demand.
[PATCH 3/4] nfsd: free unused session-DRC slots
[PATCH 4/4] nfsd: add shrinker to reduce number of slots allocated