linux-kernel - Re: [PATCH v3] block: move non sync requests complete flow to softirq

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Message-ID: <ZtfiK1xg2RVzkXW9@infradead.org>
Date: Tue, 3 Sep 2024 21:29:31 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: ZhangHui <zhanghui31@...omi.com>, bvanassche@....org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] block: move non sync requests complete flow to softirq

On Tue, Sep 03, 2024 at 11:49:28AM -0600, Jens Axboe wrote:
> The elephant in the room here is why an 80M completion takes 100 msec?
> That seems... insane.
> 
> That aside, doing writes that big isn't great for latencies in general,
> even if they are orders of magnitude smaller (as they should be). Maybe
> this is solvable by just limiting the write size here.
> 
> But it really seems out of line for a write that size to take 100 msec
> to process.

pagecache state processing is quite inefficient, we had to limit
the number of them for XFS to avoid latency problems a while ago.
Note that moving to folios means you can process a lot more data
with the same number of completion iterations as well.  I'd suggest
the submitter looks into that for whatever file system they are using.