Date: Wed, 4 Sep 2019 09:00:03 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Mark Johnston <markj@freebsd.org>, Slawa Olhovchenkov <slw@zxy.spb.ru> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r351673 - in head: lib/libmemstat share/man/man9 sys/cddl/compat/opensolaris/kern sys/kern sys/vm Message-ID: <2b4562be-46d9-7781-ac35-2fa3bfb4551f@FreeBSD.org> In-Reply-To: <20190903220106.GB26733@raichu> References: <201909012222.x81MMh0F022462@repo.freebsd.org> <79c74018-1329-ee69-3480-e2f99821fa93@FreeBSD.org> <20190903161427.GA38096@zxy.spb.ru> <20190903220106.GB26733@raichu>
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On 04/09/2019 01:01, Mark Johnston wrote: > Slawa and I talked about this in the past. His complaint is that a > large cache can take a significant amount of time to trim, and it > manifests as a spike of CPU usage and contention on the zone lock. In > particular, keg_drain() iterates over the list of free slabs with the > keg lock held, and if the many items were freed to the keg while > trimming/draining, the list can be quite long. This can have effects > outside the zone, for example if we are reclaiming items from zones used > by other UMA zones, like the bucket or slab zones. My concern is different, though. I feel that having oversized caches for long periods of time produces a skewed picture of memory usage. Particularly, some ZFS caches are sometimes extremely oversized. I don't care much about details of consequences of such oversized caches. I just think that that is not right on a more general level. > Reclaiming cached items when there is no demand for free pages seems > wrong to me. It certainly was wrong before. Now that we have a capability to trim a cache size to a workset size it doesn't feel as wrong to me. > We historically had similar problems with the page daemon, > which last year was changed to perform smaller reclamations at a greater > frequency. I suspect a better approach for UMA would be to similarly > increase reclaim frequency and reduce the number of items freed in one > go. -- Andriy Gapon
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