Re: [Linuxarm] Re: [PATCH RFC 0/7] add socket to netdev page frag recycling support

From: Eric Dumazet
Date: Wed Aug 25 2021 - 12:33:04 EST

On Wed, Aug 25, 2021 at 9:29 AM David Ahern <dsahern@xxxxxxxxx> wrote:
> On 8/23/21 8:04 AM, Eric Dumazet wrote:
> >>
> >>
> >> It seems PAGE_ALLOC_COSTLY_ORDER is mostly related to pcp page, OOM, memory
> >> compact and memory isolation, as the test system has a lot of memory installed
> >> (about 500G, only 3-4G is used), so I used the below patch to test the max
> >> possible performance improvement when making TCP frags twice bigger, and
> >> the performance improvement went from about 30Gbit to 32Gbit for one thread
> >> iperf tcp flow in IOMMU strict mode,
> >
> > This is encouraging, and means we can do much better.
> >
> > Even with SKB_FRAG_PAGE_ORDER set to 4, typical skbs will need 3 mappings
> >
> > 1) One for the headers (in skb->head)
> > 2) Two page frags, because one TSO packet payload is not a nice power-of-two.
> interesting observation. I have noticed 17 with the ZC API. That might
> explain the less than expected performance bump with iommu strict mode.

Note that if application is using huge pages, things get better after

commit 394fcd8a813456b3306c423ec4227ed874dfc08b
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Thu Aug 20 08:43:59 2020 -0700

net: zerocopy: combine pages in zerocopy_sg_from_iter()

Currently, tcp sendmsg(MSG_ZEROCOPY) is building skbs with order-0
Compared to standard sendmsg(), these skbs usually contain up to
16 fragments
on arches with 4KB page sizes, instead of two.

This adds considerable costs on various ndo_start_xmit() handlers,
especially when IOMMU is in the picture.

As high performance applications are often using huge pages,
we can try to combine adjacent pages belonging to same
compound page.

Tested on AMD Rome platform, with IOMMU, nominal single TCP flow speed
is roughly doubled (~55Gbit -> ~100Gbit), when user application
is using hugepages.

For reference, nominal single TCP flow speed on this platform
without MSG_ZEROCOPY is ~65Gbit.

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>

Ideally the gup stuff should really directly deal with hugepages, so
that we avoid
all these crazy refcounting games on the per-huge-page central refcount.

> >
> > The first issue can be addressed using a piece of coherent memory (128
> > or 256 bytes per entry in TX ring).
> > Copying the headers can avoid one IOMMU mapping, and improve IOTLB
> > hits, because all
> > slots of the TX ring buffer will use one single IOTLB slot.
> >
> > The second issue can be solved by tweaking a bit
> > skb_page_frag_refill() to accept an additional parameter
> > so that the whole skb payload fits in a single order-4 page.
> >
> >