[RFC PATCH 0/3] Introduce a bulk order-0 page allocator for sunrpc

Mel Gorman
2021-02-24T10:35:36Z

This is a prototype series that introduces a bulk order-0 page allocator
with sunrpc being the first user. The implementation is not particularly
efficient and the intention is to iron out what the semantics of the API
should be. That said, sunrpc was reported to have reduced allocation
latency when refilling a pool.

As a side-note, while the implementation could be more efficient, it
would require fairly deep surgery in numerous places. The lock scope would
need to be significantly reduced, particularly as vmstat, per-cpu and the
buddy allocator have different locking protocol that overal -- e.g. all
partially depend on irqs being disabled at various points. Secondly,
the core of the allocator deals with single pages where as both the bulk
allocator and per-cpu allocator operate in batches. All of that has to
be reconciled with all the existing users and their constraints (memory
offline, CMA and cpusets being the trickiest).

In terms of semantics required by new users, my preference is that a pair
of patches be applied -- the first which adds the required semantic to
the bulk allocator and the second which adds the new user.

Patch 1 of this series is a cleanup to sunrpc, it could be merged
separately but is included here for convenience.

Patch 2 is the prototype bulk allocator

Patch 3 is the sunrpc user. Chuck also has a patch which further caches
pages but is not included in this series. It's not directly
related to the bulk allocator and as it caches pages, it might
have other concerns (e.g. does it need a shrinker?)

This has only been lightly tested on a low-end NFS server. It did not break
but would benefit from an evaluation to see how much, if any, the headline
performance changes. The biggest concern is that a light test case showed
that there are a *lot* of bulk requests for 1 page which gets delegated to
the normal allocator. The same criteria should apply to any other users.

include/linux/gfp.h | 13 +++++
mm/page_alloc.c | 113 +++++++++++++++++++++++++++++++++++++++++-
net/sunrpc/svc_xprt.c | 47 ++++++++++++------
3 files changed, 157 insertions(+), 16 deletions(-)

--
2.26.2