On 10/11/2017 01:06 AM, Jan Kara wrote: >>> when rebasing our enterprise distro to a newer kernel (from 4.4 to 4.12) we >>> have noticed a regression in bonnie++ benchmark when deleting files. >>> Eventually we have tracked this down to a fact that page cache truncation got >>> slower by about 10%. There were both gains and losses in the above interval of >>> kernels but we have been able to identify that commit 83929372f629 "filemap: >>> prepare find and delete operations for huge pages" caused about 10% regression >>> on its own. >> It's odd that just checking if some pages are huge should be that >> expensive, but ok .. > Yeah, I was surprised as well but profiles were pretty clear on this - part > of the slowdown was caused by loads of page->_compound_head (PageTail() > and page_compound() use that) which we previously didn't have to load at > all, part was in hpage_nr_pages() function and its use. Well, page->_compound_head is part of the same cacheline as the rest of the page, and the page is surely getting touched during truncation at _some_ point. The hpage_nr_pages() might cause the cacheline to get loaded earlier than before, but I can't imagine that it's that expensive.
