Re: [PATCH v2 00/16] Multigenerational LRU Framework

Jens Axboe
2021-04-13T16:14:31Z

On 4/13/21 1:51 AM, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
>
> Hello,
>
>
> Very interesting work, thank you for sharing this :)
>
> On Tue, 13 Apr 2021 00:56:17 -0600 Yu Zhao <yuzhao@google.com> wrote:
>
>> What's new in v2
>> ================
>> Special thanks to Jens Axboe for reporting a regression in buffered
>> I/O and helping test the fix.
>
> Is the discussion open? If so, could you please give me a link?

I wasn't on the initial post (or any of the lists it was posted to), but
it's on the google page reclaim list. Not sure if that is public or not.

tldr is that I was pretty excited about this work, as buffered IO tends
to suck (a lot) for high throughput applications. My test case was
pretty simple:

Randomly read a fast device, using 4k buffered IO, and watch what
happens when the page cache gets filled up. For this particular test,
we'll initially be doing 2.1GB/sec of IO, and then drop to 1.5-1.6GB/sec
with kswapd using a lot of CPU trying to keep up. That's mainline
behavior.

The initial posting of this patchset did no better, in fact it did a bit
worse. Performance dropped to the same levels and kswapd was using as
much CPU as before, but on top of that we also got excessive swapping.
Not at a high rate, but 5-10MB/sec continually.

I had some back and forths with Yu Zhao and tested a few new revisions,
and the current series does much better in this regard. Performance
still dips a bit when page cache fills, but not nearly as much, and
kswapd is using less CPU than before.

Hope that helps,
--
Jens Axboe