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

[Pavel Begunkov]

On 20/11/2020 02:06, 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).

I see, thanks for a bit of insight

-- 
Pavel Begunkov