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

Matthew Wilcox
2021-03-25T12:52:58Z

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

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.