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
