Re: [PATCH 0/9 v6] Introduce a bulk order-0 page allocator with two in-tree users

On Thu, Mar 25, 2021 at 12:50:01PM +0000, Matthew Wilcox wrote:

On Thu, Mar 25, 2021 at 11:42:19AM +0000, Mel Gorman wrote:

quoted

This series introduces a bulk order-0 page allocator with sunrpc and
the network page pool being the first users. The implementation is not
efficient as semantics needed to be ironed out first. If no other semantic
changes are needed, it can be made more efficient.  Despite that, this
is a performance-related for users that require multiple pages for an
operation without multiple round-trips to the page allocator. Quoting
the last patch for the high-speed networking use-case

            Kernel          XDP stats       CPU     pps           Delta
            Baseline        XDP-RX CPU      total   3,771,046       n/a
            List            XDP-RX CPU      total   3,940,242    +4.49%
            Array           XDP-RX CPU      total   4,249,224   +12.68%

quoted

From the SUNRPC traces of svc_alloc_arg()

	Single page: 25.007 us per call over 532,571 calls
	Bulk list:    6.258 us per call over 517,034 calls
	Bulk array:   4.590 us per call over 517,442 calls

Both potential users in this series are corner cases (NFS and high-speed
networks) so it is unlikely that most users will see any benefit in the
short term. Other potential other users are batch allocations for page
cache readahead, fault around and SLUB allocations when high-order pages
are unavailable. It's unknown how much benefit would be seen by converting
multiple page allocation calls to a single batch or what difference it may
make to headline performance.

We have a third user, vmalloc(), with a 16% perf improvement.  I know the
email says 21% but that includes the 5% improvement from switching to
kvmalloc() to allocate area->pages.

https://lore.kernel.org/linux-mm/20210323133948.GA10046@pc638.lan/ (local)

That's fairly promising. Assuming the bulk allocator gets merged, it would
make sense to add vmalloc on top. That's for bringing it to my attention
because it's far more relevant than my imaginary potential use cases.

I don't know how many _frequent_ vmalloc users we have that will benefit
from this, but it's probably more than will benefit from improvements
to 200Gbit networking performance.

I think it was 100Gbit being looked at but your point is still valid and
there is no harm in incrementally improving over time.

-- 
Mel Gorman
SUSE Labs