From: Pavel Begunkov <asml.silence@gmail.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>,
linux-fsdevel@vger.kernel.org,
Alexander Viro <viro@zeniv.linux.org.uk>,
Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec
Date: Fri, 20 Nov 2020 02:25:08 +0000 [thread overview]
Message-ID: <e70a3c05-a968-7802-df81-0529eaa7f7b4@gmail.com> (raw)
In-Reply-To: <20201120022200.GB333150@T590>
On 20/11/2020 02:22, Ming Lei wrote:
> On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
>> On 20/11/2020 01:20, Matthew Wilcox wrote:
>>> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
>>>> The block layer spends quite a while in iov_iter_npages(), but for the
>>>> bvec case the number of pages is already known and stored in
>>>> iter->nr_segs, so it can be returned immediately as an optimisation
>>>
>>> Er ... no, it doesn't. nr_segs is the number of bvecs. Each bvec can
>>> store up to 4GB of contiguous physical memory.
>>
>> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
>> stupid statement. Thanks!
>>
>
> iov_iter_npages(bvec) still can be improved a bit by the following way:
Yep, was doing exactly that, +a couple of other places that are in my way.
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..d85ed7acce05 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1608,17 +1608,23 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
> npages = pipe_space_for_user(iter_head, pipe->tail, pipe);
> if (npages >= maxpages)
> return maxpages;
> + } else if (iov_iter_is_bvec(i)) {
> + unsigned idx, offset = i->iov_offset;
> +
> + for (idx = 0; idx < i->nr_segs; idx++) {
> + npages += DIV_ROUND_UP(i->bvec[idx].bv_len - offset,
> + PAGE_SIZE);
> + offset = 0;
> + }
> + if (npages >= maxpages)
> + return maxpages;
> } else iterate_all_kinds(i, size, v, ({
> unsigned long p = (unsigned long)v.iov_base;
> npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
> - p / PAGE_SIZE;
> if (npages >= maxpages)
> return maxpages;
> - 0;}),({
> - npages++;
> - if (npages >= maxpages)
> - return maxpages;
> - }),({
> + 0;}),0,({
> unsigned long p = (unsigned long)v.iov_base;
> npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
> - p / PAGE_SIZE;
>
--
Pavel Begunkov
next prev parent reply other threads:[~2020-11-20 2:28 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-19 23:24 [PATCH v2 0/2] optimise iov_iter Pavel Begunkov
2020-11-19 23:24 ` [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec Pavel Begunkov
2020-11-20 1:20 ` Matthew Wilcox
2020-11-20 1:39 ` Pavel Begunkov
2020-11-20 1:49 ` Matthew Wilcox
2020-11-20 1:56 ` Pavel Begunkov
2020-11-20 2:06 ` Matthew Wilcox
2020-11-20 2:08 ` Pavel Begunkov
2020-11-20 2:24 ` Ming Lei
2020-11-20 17:22 ` Pavel Begunkov
2020-11-20 17:23 ` Pavel Begunkov
2020-11-20 2:22 ` Ming Lei
2020-11-20 2:25 ` Pavel Begunkov [this message]
2020-11-20 2:54 ` Matthew Wilcox
2020-11-20 8:14 ` Christoph Hellwig
2020-11-20 12:39 ` Matthew Wilcox
2020-11-20 13:00 ` Pavel Begunkov
2020-11-20 13:13 ` Matthew Wilcox
2020-11-20 9:57 ` Pavel Begunkov
2020-11-20 13:29 ` David Laight
2020-11-19 23:24 ` [PATCH v2 2/2] iov_iter: optimise iter type checking Pavel Begunkov
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