Date: Wed, 4 Apr 2018 13:49:49 -0400 From: Mark Johnston <markj@FreeBSD.org> To: Don Lewis <truckman@FreeBSD.org> Cc: Andriy Gapon <avg@FreeBSD.org>, Bryan Drewery <bdrewery@FreeBSD.org>, Peter Jeremy <peter@rulingia.com>, Jeff Roberson <jroberson@jroberson.net>, FreeBSD current <freebsd-current@FreeBSD.org> Subject: Re: Strange ARC/Swap/CPU on yesterday's -CURRENT Message-ID: <20180404174949.GA12271@raichu> In-Reply-To: <tkrat.7e60fe1978ea51c0@FreeBSD.org> References: <20180306221554.uyshbzbboai62rdf@dx240.localdomain> <20180307103911.GA72239@kloomba> <20180311004737.3441dbf9@thor.intern.walstatt.dynvpn.de> <alpine.BSF.2.21.1803111038460.1232@desktop> <20180320070745.GA12880@server.rulingia.com> <2b3db2af-03c7-65ff-25e7-425cfd8815b6@FreeBSD.org> <1fd2b47b-b559-69f8-7e39-665f0f599c8f@FreeBSD.org> <tkrat.9bab32187c0e8d01@FreeBSD.org> <tkrat.11e402b8455bd0fa@FreeBSD.org> <tkrat.7e60fe1978ea51c0@FreeBSD.org>
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On Tue, Apr 03, 2018 at 09:42:48PM -0700, Don Lewis wrote:
> On 3 Apr, Don Lewis wrote:
> > I reconfigured my Ryzen box to be more similar to my default package
> > builder by disabling SMT and half of the RAM, to limit it to 8 cores
> > and 32 GB and then started bisecting to try to track down the problem.
> > For each test, I first filled ARC by tarring /usr/ports/distfiles to
> > /dev/null. The commit range that I was searching was r329844 to
> > r331716. I narrowed the range to r329844 to r329904. With r329904
> > and newer, ARC is totally unresponsive to memory pressure and the
> > machine pages heavily. I see ARC sizes of 28-29GB and 30GB of wired
> > RAM, so there is not much leftover for getting useful work done. Active
> > memory and free memory both hover under 1GB each. Looking at the
> > commit logs over this range, the most likely culprit is:
> >
> > r329882 | jeff | 2018-02-23 14:51:51 -0800 (Fri, 23 Feb 2018) | 13 lines
> >
> > Add a generic Proportional Integral Derivative (PID) controller algorithm and
> > use it to regulate page daemon output.
> >
> > This provides much smoother and more responsive page daemon output, anticipating
> > demand and avoiding pageout stalls by increasing the number of pages to match
> > the workload. This is a reimplementation of work done by myself and mlaier at
> > Isilon.
> >
> >
> > It is quite possible that the recent fixes to the PID controller will
> > fix the problem. Not that r329844 was trouble free ... I left tar
> > running over lunchtime to fill ARC and the OOM killer nuked top, tar,
> > ntpd, both of my ssh sessions into the machine, and multiple instances
> > of getty while I was away. I was able to log in again and successfully
> > run poudriere, and ARC did respond to the memory pressure and cranked
> > itself down to about 5 GB by the end of the run. I did not see the same
> > problem with tar when I did the same with r329904.
>
> I just tried r331966 and see no improvement. No OOM process kills
> during the tar run to fill ARC, but with ARC filled, the machine is
> thrashing itself at the start of the poudriere run while trying to build
> ports-mgmt/pkg (39 minutes so far). ARC appears to be unresponsive to
> memory demand. I've seen no decrease in ARC size or wired memory since
> starting poudriere.
Re-reading the ARC reclaim code, I see a couple of issues which might be
at the root of the behaviour you're seeing.
1. zfs_arc_free_target is too low now. It is initialized to the page
daemon wakeup threshold, which is slightly above v_free_min. With the
PID controller, the page daemon uses a setpoint of v_free_target.
Moreover, it now wakes up regularly rather than having wakeups be
synchronized by a mutex, so it will respond quickly if the free page
count dips below v_free_target. The free page count will dip below
zfs_arc_free_target only in the face of sudden and extreme memory
pressure now, so the FMT_LOTSFREE case probably isn't getting
exercised. Try initializing zfs_arc_free_target to v_free_target.
2. In the inactive queue scan, we used to compute the shortage after
running uma_reclaim() and the lowmem handlers (which includes a
synchronous call to arc_lowmem()). Now it's computed before, so we're
not taking into account the pages that get freed by the ARC and UMA.
The following rather hacky patch may help. I note that the lowmem
logic is now somewhat broken when multiple NUMA domains are
configured, however, since it fires only when domain 0 has a free
page shortage.
Index: sys/vm/vm_pageout.c
===================================================================
--- sys/vm/vm_pageout.c (revision 331933)
+++ sys/vm/vm_pageout.c (working copy)
@@ -1114,25 +1114,6 @@
boolean_t queue_locked;
/*
- * If we need to reclaim memory ask kernel caches to return
- * some. We rate limit to avoid thrashing.
- */
- if (vmd == VM_DOMAIN(0) && pass > 0 &&
- (time_uptime - lowmem_uptime) >= lowmem_period) {
- /*
- * Decrease registered cache sizes.
- */
- SDT_PROBE0(vm, , , vm__lowmem_scan);
- EVENTHANDLER_INVOKE(vm_lowmem, VM_LOW_PAGES);
- /*
- * We do this explicitly after the caches have been
- * drained above.
- */
- uma_reclaim();
- lowmem_uptime = time_uptime;
- }
-
- /*
* The addl_page_shortage is the number of temporarily
* stuck pages in the inactive queue. In other words, the
* number of pages from the inactive count that should be
@@ -1824,6 +1805,26 @@
atomic_store_int(&vmd->vmd_pageout_wanted, 1);
/*
+ * If we need to reclaim memory ask kernel caches to return
+ * some. We rate limit to avoid thrashing.
+ */
+ if (vmd == VM_DOMAIN(0) &&
+ vmd->vmd_free_count < vmd->vmd_free_target &&
+ (time_uptime - lowmem_uptime) >= lowmem_period) {
+ /*
+ * Decrease registered cache sizes.
+ */
+ SDT_PROBE0(vm, , , vm__lowmem_scan);
+ EVENTHANDLER_INVOKE(vm_lowmem, VM_LOW_PAGES);
+ /*
+ * We do this explicitly after the caches have been
+ * drained above.
+ */
+ uma_reclaim();
+ lowmem_uptime = time_uptime;
+ }
+
+ /*
* Use the controller to calculate how many pages to free in
* this interval.
*/
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