Re: [RFC PATCH 00/30] Code tagging framework and applications

Jens Axboe
2022-09-02T19:54:09Z

On 9/2/22 1:48 PM, Kent Overstreet wrote:
> On Fri, Sep 02, 2022 at 06:02:12AM -0600, Jens Axboe wrote:
>> On 9/1/22 7:04 PM, Roman Gushchin wrote:
>>> On Thu, Sep 01, 2022 at 08:17:47PM -0400, Kent Overstreet wrote:
>>>> On Thu, Sep 01, 2022 at 03:53:57PM -0700, Roman Gushchin wrote:
>>>>> I'd suggest to run something like iperf on a fast hardware. And maybe some
>>>>> io_uring stuff too. These are two places which were historically most sensitive
>>>>> to the (kernel) memory accounting speed.
>>>>
>>>> I'm getting wildly inconsistent results with iperf.
>>>>
>>>> io_uring-echo-server and rust_echo_bench gets me:
>>>> Benchmarking: 127.0.0.1:12345
>>>> 50 clients, running 512 bytes, 60 sec.
>>>>
>>>> Without alloc tagging: 120547 request/sec
>>>> With: 116748 request/sec
>>>>
>>>> https://github.com/frevib/io_uring-echo-server
>>>> https://github.com/haraldh/rust_echo_bench
>>>>
>>>> How's that look to you? Close enough? :)
>>>
>>> Yes, this looks good (a bit too good).
>>>
>>> I'm not that familiar with io_uring, Jens and Pavel should have a better idea
>>> what and how to run (I know they've workarounded the kernel memory accounting
>>> because of the performance in the past, this is why I suspect it might be an
>>> issue here as well).
>>
>> io_uring isn't alloc+free intensive on a per request basis anymore, it
>> would not be a good benchmark if the goal is to check for regressions in
>> that area.
>
> Good to know. The benchmark is still a TCP benchmark though, so still useful.
>
> Matthew suggested
> while true; do echo 1 >/tmp/foo; rm /tmp/foo; done
>
> I ran that on tmpfs, and the numbers with and without alloc tagging were
> statistically equal - there was a fair amount of variation, it wasn't a super
> controlled test, anywhere from 38-41 seconds with 100000 iterations (and alloc
> tagging was some of the faster runs).
>
> But with memcg off, it ran in 32-33 seconds. We're piggybacking on the same
> mechanism memcg uses for stashing per-object pointers, so it looks like that's
> the bigger cost.

I've complained about memcg accounting before, the slowness of it is why
io_uring works around it by caching. Anything we account we try NOT do
in the fast path because of it, the slowdown is considerable.

You care about efficiency now? I thought that was relegated to
irrelevant 10M IOPS cases.

--
Jens Axboe