Re: The pagedaemon evicts ARC before scanning the inactive page list
Date: Wed, 19 May 2021 03:55:25 UTC
On Tue, May 18, 2021 at 9:25 PM Konstantin Belousov <kostikbel@gmail.com> wrote: > On Tue, May 18, 2021 at 05:55:36PM -0600, Alan Somers wrote: > > On Tue, May 18, 2021 at 4:10 PM Mark Johnston <markj@freebsd.org> wrote: > > > > > On Tue, May 18, 2021 at 04:00:14PM -0600, Alan Somers wrote: > > > > On Tue, May 18, 2021 at 3:45 PM Mark Johnston <markj@freebsd.org> > wrote: > > > > > > > > > On Tue, May 18, 2021 at 03:07:44PM -0600, Alan Somers wrote: > > > > > > I'm using ZFS on servers with tons of RAM and running FreeBSD > > > > > > 12.2-RELEASE. Sometimes they get into a pathological situation > where > > > > > most > > > > > > of that RAM sits unused. For example, right now one of them has: > > > > > > > > > > > > 2 GB Active > > > > > > 529 GB Inactive > > > > > > 16 GB Free > > > > > > 99 GB ARC total > > > > > > 469 GB ARC max > > > > > > 86 GB ARC target > > > > > > > > > > > > When a server gets into this situation, it stays there for days, > > > with the > > > > > > ARC target barely budging. All that inactive memory never gets > > > reclaimed > > > > > > and put to a good use. Frequently the server never recovers > until a > > > > > reboot. > > > > > > > > > > > > I have a theory for what's going on. Ever since r334508^ the > > > pagedaemon > > > > > > sends the vm_lowmem event _before_ it scans the inactive page > list. > > > If > > > > > the > > > > > > ARC frees enough memory, then vm_pageout_scan_inactive won't > need to > > > free > > > > > > any. Is that order really correct? For reference, here's the > > > relevant > > > > > > code, from vm_pageout_worker: > > > > > > > > > > That was the case even before r334508. Note that prior to that > > > revision > > > > > vm_pageout_scan_inactive() would trigger vm_lowmem if pass > 0, > before > > > > > scanning the inactive queue. During a memory shortage we have > pass > > > > 0. > > > > > pass == 0 only when the page daemon is scanning the active queue. > > > > > > > > > > > shortage = pidctrl_daemon(&vmd->vmd_pid, vmd->vmd_free_count); > > > > > > if (shortage > 0) { > > > > > > ofree = vmd->vmd_free_count; > > > > > > if (vm_pageout_lowmem() && vmd->vmd_free_count > ofree) > > > > > > shortage -= min(vmd->vmd_free_count - ofree, > > > > > > (u_int)shortage); > > > > > > target_met = vm_pageout_scan_inactive(vmd, shortage, > > > > > > &addl_shortage); > > > > > > } else > > > > > > addl_shortage = 0 > > > > > > > > > > > > Raising vfs.zfs.arc_min seems to workaround the problem. But > ideally > > > > > that > > > > > > wouldn't be necessary. > > > > > > > > > > vm_lowmem is too primitive: it doesn't tell subscribing subsystems > > > > > anything about the magnitude of the shortage. At the same time, > the VM > > > > > doesn't know much about how much memory they are consuming. A > better > > > > > strategy, at least for the ARC, would be reclaim memory based on > the > > > > > relative memory consumption of each subsystem. In your case, when > the > > > > > page daemon goes to reclaim memory, it should use the inactive > queue to > > > > > make up ~85% of the shortfall and reclaim the rest from the ARC. > Even > > > > > better would be if the ARC could use the page cache as a > second-level > > > > > cache, like the buffer cache does. > > > > > > > > > > Today I believe the ARC treats vm_lowmem as a signal to shed some > > > > > arbitrary fraction of evictable data. If the ARC is able to > quickly > > > > > answer the question, "how much memory can I release if asked?", > then > > > > > the page daemon could use that to determine how much of its > reclamation > > > > > target should come from the ARC vs. the page cache. > > > > > > > > > > > > > I guess I don't understand why you would ever free from the ARC > rather > > > than > > > > from the inactive list. When is inactive memory ever useful? > > > > > > Pages in the inactive queue are either unmapped or haven't had their > > > mappings referenced recently. But they may still be frequently > accessed > > > by file I/O operations like sendfile(2). That's not to say that > > > reclaiming from other subsystems first is always the right strategy, > but > > > note also that the page daemon may scan the inactive queue many times > in > > > between vm_lowmem calls. > > > > > > > So By default ZFS tries to free (arc_target / 128) bytes of memory in > > arc_lowmem. That's huge! On this server, pidctrl_daemon typically > > requests 0-10MB, and arc_lowmem tries to free 600 MB. It looks like it > > would be easy to modify vm_lowmem to include the total amount of memory > > that it wants freed. I could make such a patch. My next question is: > > what's the fastest way to generate a lot of inactive memory? My first > > attempt was "find . | xargs md5", but that isn't terribly effective. The > > production machines are doing a lot of "zfs recv" and running some busy > Go > > programs, among other things, but I can't easily replicate that workload > on > > Is your machine ZFS-only? If yes, then typical source of inactive memory > can be of two kinds: > No, there is also FUSE. But there is typically < 1GB of Buf memory, so I didn't mention it. > - anonymous memory that apps allocate with facilities like malloc(3). > If inactive is shrinkable then it is probably not, because dirty pages > from anon objects must go through laundry->swap route to get evicted, > and you did not mentioned swapping > No, there's no appreciable amount of swapping going on. Nor is the laundry list typically more than a few hundred MB. > - double-copy pages cached in v_objects of ZFS vnodes, clean or dirty. > If unmapped, these are mostly a waste. Even if mapped, the source > of truth for data is ARC, AFAIU, so they can be dropped as well, since > inactive state means that its content is not hot. > So if a process mmap()'s a file on ZFS and reads from it but never writes to it, will those pages show up as inactive? > > You can try to inspect the most outstanding objects adding to the > inactive queue with 'vmobject -o' to see where the most of inactive pages > come from. > Wow, that did it! About 99% of the inactive pages come from just a few vnodes which are used by the FUSE servers. But I also see a few large entries like 1105308 333933 771375 1 0 WB df what does that signify? > > If indeed they are double-copy, then perhaps ZFS can react even to the > current primitive vm_lowmem signal somewhat different. First, it could > do the pass over its vnodes and > - free clean unmapped pages > - if some targets are not met after that, laundry dirty pages, > then return to freeing clean unmapped pages > all that before ever touching its cache (ARC). >