Date: Tue, 24 Nov 2015 20:32:36 -0700 From: Ian Lepore <ian@freebsd.org> To: John Baldwin <jhb@freebsd.org>, freebsd-hackers@freebsd.org Subject: Re: vmstat -m strangeness Message-ID: <1448422356.1297.5.camel@freebsd.org> In-Reply-To: <1769220.C3nP8qXIrX@ralph.baldwin.cx> References: <1445378386.14127.2.camel@freebsd.org> <1769220.C3nP8qXIrX@ralph.baldwin.cx>
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On Tue, 2015-11-24 at 17:10 -0800, John Baldwin wrote: > On Tuesday, October 20, 2015 03:59:46 PM Ian Lepore wrote: > > root@wand:~ # vmstat -m | egrep "busdma|bounce|devbuf|Type" > > Type InUse MemUse HighUse Requests Size(s) > > devbuf 125 10K - 166 16,32,64,256,512,1024 > > busdma 922 116K - 922 128 > > bounce 385 775K - 385 32,128 > > > > How do 385 allocations of 32 or 128 bytes add up to 775K? The > > answer... 768K of individual pages each allocated via > > contigmalloc() do > > n't show up in that output. Why is that, and is it something that > > should be fixed? > > Hmm. The output is "correct", it's just that these small allocations > have > relatively high overhead (PAGE_SIZE - 32 for a 32 byte allocation, > etc.). > > Not sure if there's a way to explain (in vmstat output) why the > overhead for > certain malloc buckets is so high. Perhaps the vmstat(8) manpage > could be > expanded to explain what MemUse is and list the contigmalloc() case > explicitly > as one possible reason for high overhead? > I think I wasn't very clear. There are 385 small allocations (mostly 32 byte) which are using 7 pages. The other 768 pages were allocated by calling contigmalloc() 768 times asking for one page. The strangeness is that those pages aren't reflected in that output. There's no '4096' category in sizes. To put it another way, contigmalloc() is accounting for the memory it allocates in a way that makes it show up in MemUse but not in Size(s), which makes for confusing output. No big deal really, just something I noticed. -- Ian
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