linux-kernel - Re: [git pull] iov

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Message-ID: <b8786a7e-5616-ce83-c2f2-53a4754bf5a4@kernel.dk>
Date:   Thu, 9 Sep 2021 21:05:13 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] iov_iter fixes

On 9/9/21 8:57 PM, Al Viro wrote:
> On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote:
> 
>> Not sure how we'd do that, outside of stupid tricks like copy the
>> iov_iter before we pass it down. But that's obviously not going to be
>> very efficient. Hence we're left with having some way to reset/reexpand,
>> even in the presence of someone having done truncate on it.
> 
> "Obviously" why, exactly?  It's not that large a structure; it's not
> the optimal variant, but I'd like to see profiling data before assuming
> that it'll cause noticable slowdowns.

It's 48 bytes, and we have to do it upfront. That means we'd be doing it
for _all_ requests, not just when we need to retry. As an example, current
benchmarks are at ~4M read requests per core. That'd add ~200MB/sec of
memory traffic just doing this copy.

Besides, I think that's moot as there's a better way.

-- 
Jens Axboe