Date: Mon, 18 May 2009 13:31:10 -0400 From: John Baldwin <jhb@freebsd.org> To: Attilio Rao <attilio@freebsd.org> Cc: Adam McDougall <mcdouga9@egr.msu.edu>, freebsd-current@freebsd.org, Artem Belevich <fbsdlist@src.cx>, Ben Kelly <ben@wanderview.com> Subject: Re: [patch] zfs livelock and thread priorities Message-ID: <200905181331.11174.jhb@freebsd.org> In-Reply-To: <3bbf2fe10905181012t4bde260bp31181e3ea7b03a42@mail.gmail.com> References: <08D7DC2A-68BE-47B6-8D5D-5DE6B48F87E5@wanderview.com> <200905181129.51526.jhb@freebsd.org> <3bbf2fe10905181012t4bde260bp31181e3ea7b03a42@mail.gmail.com>
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On Monday 18 May 2009 1:12:59 pm Attilio Rao wrote: > 2009/5/18 John Baldwin <jhb@freebsd.org>: > > On Saturday 16 May 2009 12:40:44 pm Ben Kelly wrote: > >> 1) It changes the kproc(9) API by adding a kproc_create_priority() > >> function that allows you to set the priority of the newly created > >> thread. I'm not sure how people feel about this. > > > > Actually, I almost think we should just add a priority argument to each of the > > routines that creates a new kthread/kproc. Perhaps allow a priority of 0 to > > let the thread run with the default priority. Hmm, it looks like kthreads > > default to running with whatever thread0 runs at (PVM) which is probably not > > really ideal. Having an explicit priority for every kthread would probably > > be best. Most kthreads should probably be at PZERO by default I think. > > I'm not sure I agree here. > 1) Maybe I missed it (so please point me to the right one) but I > didn't see a deep analysis of what messed up with the priorities there Solaris makes certain assumptions about the relative priorities of ZFS threads and our ZFS doesn't set the priorities the same. I think specifically there are "cleaner" threads which have a higher priority on Solaris than other ZFS threads and Solaris depends on that to avoid deadlocks. > 2) I think this KPI can be dangerous and lead to problems. Priority is > something highly fragile. All the more reason to make developers _think_ about the priority of each kthread they create. Right now all these threads start out with a priority of PVM since that is what thread0 runs at. Does that sound right to you? Do you think many folks realize that? It sounds very bogus to me. I think forcing people to pick a sensible priority for each thread is far better than the complete lack of thought that often happens now. -- John Baldwin
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