Date: Tue, 8 Dec 2015 09:55:54 +0100 From: Peter Holm <peter@holm.cc> To: Gleb Smirnoff <glebius@FreeBSD.org> Cc: jeff@FreeBSD.org, alc@FreeBSD.org, kib@FreeBSD.org, scottl@FreeBSD.org, arch@FreeBSD.org Subject: Re: new vm_pager_get_pages() KPI, round 3 Message-ID: <20151208085554.GA60158@x2.osted.lan> In-Reply-To: <20151205052940.GJ42565@FreeBSD.org> References: <20151205052940.GJ42565@FreeBSD.org>
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On Sat, Dec 05, 2015 at 08:29:40AM +0300, Gleb Smirnoff wrote: > Hi, > > [first paragraph for arch subscribers, To: recepients may skip] > > This patch is kinda a prerequisite for the non-blocking sendfile(2), > that was jointly developed by NGINX and Netflix in 2014 and has been > running in Netflix production for a year, serving 35% of the whole > North America (US, Canada, Mexico) Internet traffic. > Technically, the new sendfile(2) doesn't require the new > vm_pager_get_pages() KPI. We currently run it on the old KPI. However, > kib@ suggested that we are abusing the KPI, carefully using its > edge cases. To address this critic, back in spring, I suggested a KPI, > where vm_pager_get_pages() offers all-or-none approach to the array of > pages. Again, kib@ wasn't satisfied, as for "the main user" of > vm_pager_get_pages, the vm_fault(), all-or-none approach isn't optimal. > The problem was slowly debated through the summer. And then in October > jeff@ suggested yet another extension of the KPI, which I have > implemented and it is described below. > > [for those interested in new sendfile(2), skip to the last paragraph, > for those willing to review new pager KPI, read on] > > The new KPI offers this prototype for vm_pager_get_pages(): > > int > vm_pager_get_pages(vm_object_t object, vm_page_t pages[], int count, > int *rbehind, in *rahead); > > Where "count" stands for number of pages in the array. The rbehind > and rahead if not NULL specify how many pages the caller is willing to > allow the pager to pre-cache, if the pager can. > > Pager doesn't promise to do any read behind or read ahead. If it does, > then only the pager is responsive for grabbing, busying, unbusying and > queueing these pages. It also writes the actual values of completed > read ahead and read behind back to the pointers. > > Pager promises to page in "count" pages or fail. Pager expects the > pages to be busied, and returns them busied. For a multi page requests, > the pager demands that the region is a valid region, that exists in > the pager, which can be checked by preceding call to vm_pager_haspage(). > For single page requests, there is no such demand. > > The net result is a win for both vm_fault() and for new sendfile(). > > The vm_fault() no longer needs to do prepatory vm_pager_haspage(), > which removes one I/O operation. The logic for read ahead/behind, > which is strongly UFS/EXT-centric, moves into vnode_pager.c. So > we no longer do useless operations when having a fault on ZFS. > > The vm_fault() now knows precisely the read ahead that happened, > when updates fs.entry->next_read index. This reduces number of > hardfaults by a tiny fraction (measured building world tree). > > The new sendfile() has a stronger KPI, that doesn't unbusy pages, > that sendfile() needs to be kept busied. > > Also, the new KPI removes some ugly edges. E.g., since the old > KPI used to unbusy and free pages in the array in case of an > error, the pages could not be wired. However, there are places in > kernel where we want to page in into a wired page. These places > simply violated the assumption, relying on lack of errors in the > pager. Moreover, the swap pager has a special function to skip > wired pages, while doing the freeing sweep, to avoid hitting > assertion. That means passing wired pages to swapper is kinda > OK, while to any other pager it is not. So, we end up with > vm_pager_get_pages() being not pager agnostic, while it is > designed to be so. Now this is fixed. > > Peter, if you can, please try the patch in your tests. I already > did that, but you are always better at this :) > I ran all test that I have on amd64 / 24 CPU and 64GB. Ran a "-j 25" buildworld on amd64 / 24 CPU and 2GB. Ran a buildkernel on i386 / 1 CPU and 256MB. No problems seen. - Peter
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