Date: Wed, 30 Apr 2008 21:57:36 -0700 From: Alfred Perlstein <alfred@freebsd.org> To: gnn@freebsd.org Cc: stable@freebsd.org Subject: Re: Some odd behaviour of vmstat and vmtotal... Message-ID: <20080501045736.GU30325@elvis.mu.org> In-Reply-To: <m2lk2ultc1.wl%gnn@neville-neil.com> References: <m2lk2ultc1.wl%gnn@neville-neil.com>
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* gnn@freebsd.org <gnn@freebsd.org> [080430 18:13] wrote: > Howdy, > > In deploying 7.0 at work we were finding a persistent problem when > running "vmstat 1" on systems. The problem shows up as a 10ms "pause" > in processing, usually packet stamping and forwarding by a user level > process. This is due to the fact that vmstat calls the vmtotal() > routine which in turn does a whole lot of locking. The vmtotal call > locks and walks the VM object list twice in a mark and sweep > operation. > > So, the question is, "What is the right way to fix this?" I could > remove the locking since the O_ACTIVE bit is not used by any other > routine besides vmtotal, but I'm not too happy about that. > > The relevant code can be found in src/sys/vm/vm_meter.c in vmtotal(). > > Thoughts? Can you _really_ remove the locking? I don't think that would be safe if the list is being manipulated, but I haven't checked. Typically one uses a sentinal to get around such problems. Basically insert a dummy object that no one should touch other than you, then drop the lock periodically and insert it into the list, then resume. -- - Alfred Perlstein
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