Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

[Pavel Begunkov]

On 20/11/2020 02:24, Ming Lei wrote:
> On Fri, Nov 20, 2020 at 02:06:10AM +0000, Matthew Wilcox wrote:
>> On Fri, Nov 20, 2020 at 01:56:22AM +0000, Pavel Begunkov wrote:
>>> On 20/11/2020 01:49, Matthew Wilcox 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!
>>>>>
>>>>> Are there many users of that? All these iterators are a huge burden,
>>>>> just to count one 4KB page in bvec it takes 2% of CPU time for me.
>>>>
>>>> __bio_try_merge_page() will create multipage BIOs, and that's
>>>> called from a number of places including
>>>> bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages()
>>>
>>> I get it that there are a lot of places, more interesting how often
>>> it's actually triggered and if that's performance critical for anybody.
>>> Not like I'm going to change it, just out of curiosity, but bvec.h
>>> can be nicely optimised without it.
>>
>> Typically when you're allocating pages for the page cache, they'll get
>> allocated in order and then you'll read or write them in order, so yes,
>> it ends up triggering quite a lot.  There was once a bug in the page
>> allocator which caused them to get allocated in reverse order and it
>> was a noticable performance hit (this was 15-20 years ago).
> 
> hugepage use cases can benefit much from this way too.

This didn't yield any considerable boost for me though. 1.5% -> 1.3%
for 1 page reads. I'll send it anyway though because there are cases
that can benefit, e.g. as Ming mentioned.

Ming would you want to send the patch yourself? After all you did post
it first.

-- 
Pavel Begunkov