On 9/10/24 11:44, Yunsheng Lin wrote:
On 2024/9/10 0:54, Mina Almasry wrote:
On Mon, Sep 9, 2024 at 4:21 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
On 2024/9/9 13:43, Mina Almasry wrote:
Perf - page-pool benchmark:
---------------------------
bench_page_pool_simple.ko tests with and without these changes:
https://pastebin.com/raw/ncHDwAbn
AFAIK the number that really matters in the perf tests is the
'tasklet_page_pool01_fast_path Per elem'. This one measures at about 8
cycles without the changes but there is some 1 cycle noise in some
results.
With the patches this regresses to 9 cycles with the changes but there
is 1 cycle noise occasionally running this test repeatedly.
Lastly I tried disable the static_branch_unlikely() in
netmem_is_net_iov() check. To my surprise disabling the
static_branch_unlikely() check reduces the fast path back to 8 cycles,
but the 1 cycle noise remains.
Sorry for the late report, as I was adding a testing page_pool ko basing
on [1] to avoid introducing performance regression when fixing the bug in
[2].
I used it to test the performance impact of devmem patchset for page_pool
too, it seems there might be some noticable performance impact quite stably
for the below testcases, about 5%~16% performance degradation as below in
the arm64 system:
Correct me if I'm wrong here, but on the surface here it seems that
you're re-reporting a known issue. Consensus seems to be that it's a
non-issue.
In v6 I reported that the bench_page_pool_simple.ko test reports a 1
cycle regression with these patches, from 8->9 cycles. That is roughly
consistent with the 5-15% you're reporting.
From the description above in the cover letter, I thought the performance
data using the out of tree testing ko is not stable enough to justify the
performance impact.
I root caused the reason for the regression to be the
netmem_is_net_iov() check in the fast path. I removed this regression
in v7 (see the change log) by conditionally compiling the check in
that function.
In v8, Pavel/Jens/David pushed back on the ifdef check. See this
entire thread, but in particular this response from Jens:
It seemed the main objection is about how to enable this feature
for the io_uring?
The pushback was that config checks as optimisation don't work in real
life, they inevitably get enabled everywhere but some niche cases.
io_uring could do another config for memory providers, but even if it's
not enabled by default (which is not a great option), distributions will
eventually turn it on.
So, if you have that "niche use case" that fully controls the kernel and
wants to shed this overhead, we can do a config structure, but if it's
about overhead for everyone in general, configs hardly help anything,
even without any io_uring in the picture.
And it seemed that you had added the CONFIG_NET_DEVMEM for this
devmem thing, why not use it for that?
--
Pavel Begunkov
