From: Pavel Begunkov <asml.silence@gmail.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] block: optimise bvec_iter_advance()
Date: Sun, 1 Dec 2019 03:04:36 +0300 [thread overview]
Message-ID: <74c78d40-8b0c-2e87-d882-8396411a7be5@gmail.com> (raw)
In-Reply-To: <20191130220458.GA297712@rani.riverdale.lan>
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On 01/12/2019 01:05, Arvind Sankar wrote:
> On Sat, Nov 30, 2019 at 11:23:52PM +0300, Pavel Begunkov wrote:
>> bvec_iter_advance() is quite popular, but compilers fail to do proper
>> alias analysis and optimise it good enough. The assembly is checked
>> for gcc 9.2, x86-64.
>>
>> - remove @iter->bi_size from min(...), as it's always less than @bytes.
>> Modify at the beginning and forget about it.
>>
>> - the compiler isn't able to collapse memory dependencies and remove
>> writes in the loop. Help it by explicitly using local vars.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
>>
>> v2: simplify code (Arvind Sankar)
>>
>
> Thanks :)
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
>
> Btw, I discovered that gcc 9.2 doesn't optimize away the second
> comparison in something like
That's unfortunate. IDK, whether it could be easily done in gcc,
but maybe relay it to compiler guys?
>
> m = min(a,b);
> return m>a;
>
> So the WARN_ONCE bit doesn't get optimized away even in cases like
> bio_for_each_bvec where it's guaranteed at compile-time to not trigger.
>
That's more like a starting point. The idea is to revise and
tune/rewrite iteration helpers including *for_each_bvec(). I assume,
all those are really poorly optimisable (especially with
-fno-strict-aliasing). So, noted to consider
--
Pavel Begunkov
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next prev parent reply other threads:[~2019-12-01 0:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1575144884.git.asml.silence@gmail.com>
2019-11-30 20:23 ` [PATCH v2] block: optimise bvec_iter_advance() Pavel Begunkov
2019-11-30 22:05 ` Arvind Sankar
2019-12-01 0:04 ` Pavel Begunkov [this message]
2019-12-03 14:37 ` Jens Axboe
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