Date: Tue, 28 Sep 2010 16:36:41 +0300 From: Andriy Gapon <avg@icyb.net.ua> To: Jeremy Chadwick <freebsd@jdc.parodius.com> Cc: stable@freebsd.org, Willem Jan Withagen <wjw@digiware.nl>, fs@freebsd.org Subject: Re: Still getting kmem exhausted panic Message-ID: <4CA1EF69.4040402@icyb.net.ua> In-Reply-To: <20100928132355.GA63149@icarus.home.lan> References: <4CA1D06C.9050305@digiware.nl> <20100928115047.GA62142__15392.0458550148$1285675457$gmane$org@icarus.home.lan> <4CA1DDE9.8090107@icyb.net.ua> <20100928132355.GA63149@icarus.home.lan>
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on 28/09/2010 16:23 Jeremy Chadwick said the following: > On Tue, Sep 28, 2010 at 03:22:01PM +0300, Andriy Gapon wrote: >> on 28/09/2010 14:50 Jeremy Chadwick said the following: >>> I believe the trick -- Andriy, please correct me if I'm wrong -- is the >> >> Wouldn't hurt to CC me, so that I could do it :-) >> >>> tuning of vfs.zfs.arc_max, which is now a hard limit rather than a "high >>> watermark". >> >> Not sure what you mean here. >> What is hard limit, what is high watermark, what is the difference and when is >> "now"? :-) > > There was some speculation on the part of users a while back which lead > to this understanding. Folks were seeing actual ARC usage higher than > what vfs.zfs.arc_max was set to (automatically or administratively). I > believe it started here: > > http://www.mailinglistarchive.com/freebsd-current@freebsd.org/msg28884.html > > With the "high-water mark" statements being here: > > http://www.mailinglistarchive.com/freebsd-current@freebsd.org/msg28887.html > http://unix.derkeiler.com/Mailing-Lists/FreeBSD/stable/2010-04/msg00129.html > > The term implies that there is not an explicitly hard limit on the ARC > utilisation/growth. As stated in the unix.derkeiler.com URL above, this > behaviour was in fact changed. Why/when/how? I had to go digging up > the commits -- this took me some time. Here they are, labelled r197816, > for RELENG_8 and RELENG_7 respectively. These were both committed on > 2010/01/08 UTC: > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c#rev1.22.2.2 > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c#rev1.15.2.6 > > In HEAD/CURRENT (yet to be MFC'd), it looks like above code got removed > on 2010/09/17 UTC, citing they should be "enforced by actual > calculations of delta": > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c#rev1.46 > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c#rev1.45 > > So what's this "delta" code piece that's mentioned? That appears to be > have been committed to RELENG_8 on 2010/05/24 UTC (thus, between the > above two dates): > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c#rev1.22.2.4 > > (Side note: the "delta stuff" was never committed to RELENG_7 -- and > that's fine. I'm pointing this out not out of retaliation or insult, > but because people will almost certainly Google, find this post, and > wonder if their 7.x machines might be affected.) > > This situation with the ARC, and all its changes over time, is one of > the reasons why I rant aggressively about the need for more > communication transparency (re: what the changes actually affect). Most > SAs and users don't follow commits. Well, no time for me to dig through all that history. arc_max should be a hard limit and it is now. If it ever wasn't then it was a bug. Besides, "high watermark" is still an ambiguous term, for you it "implies" that it is not a hard limit, but for me it "implies" exactly a hard limit. Additionally, going from "non-hard limit" to a "hard limit" on ARC size should improve things memory-wise, not vice versa, right? :) P.S. All that I said above is a hint that this is a pointless branch of the thread :) -- Andriy Gapon
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