Date: Tue, 5 Mar 2013 21:16:13 -0800 From: Jeremy Chadwick <jdc@koitsu.org> To: Karl Denninger <karl@denninger.net> Cc: freebsd-stable@freebsd.org Subject: Re: ZFS "stalls" -- and maybe we should be talking about defaults? Message-ID: <20130306051613.GA62470@icarus.home.lan> In-Reply-To: <20130306050809.GA61727@icarus.home.lan> References: <513524B2.6020600@denninger.net> <89680320E0FA4C0A99D522EA2037CE6E@multiplay.co.uk> <20130305050539.GA52821@anubis.morrow.me.uk> <20130305053249.GA38107@icarus.home.lan> <545CD2ABE3D146F2B91963ADF6090CDE@multiplay.co.uk> <20130305092700.GA43045@icarus.home.lan> <5135EB62.6060006@denninger.net> <20130306050809.GA61727@icarus.home.lan>
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On Tue, Mar 05, 2013 at 09:08:09PM -0800, Jeremy Chadwick wrote:
> > > * How long the stall is in duration (ex. if there's some way to
> > > roughly calculate this using "date" in a shell script)
> > They're variable. Some last fractions of a second and are not really
> > all that noticeable unless you happen to be paying CLOSE attention.
> > Some last a few (5 or so) seconds. The really bad ones last long enough
> > that the kernel throws the message "swap_pager: indefinite wait buffer".
>
> The message "swap_pager: indefinite wait buffer" indicates that some
> part of the VM is trying to offload pages of memory to swap via standard
> I/O write requests, and those writes have not come back within kern.hz*20
> seconds. That's a very, very long time.
Two clarification points:
1. The timeout value is passed to msleep(9) and is literally kern.hz*20.
Per sys/vm/swap_pager.c:
1216 if (msleep(mreq, VM_OBJECT_MTX(object), PSWP, "swread", hz*20)) {
1217 printf(
1218 "swap_pager: indefinite wait buffer: bufobj: %p, blkno: %jd, size: %ld\n",
1219 bp->b_bufobj, (intmax_t)bp->b_blkno, bp->b_bcount);
How that's interpreted is documented in msleep(9):
The parameter timo specifies a timeout for the sleep. If timo is
not 0, then the thread will sleep for at most timo / hz seconds.
If the timeout expires, then the sleep function will return
EWOULDBLOCK.
2. The message appears to be for swap I/O *reads*, not writes; at least
that's what the "swread" STATE string (you know, what you see in
top(1)) implies.
--
| Jeremy Chadwick jdc@koitsu.org |
| UNIX Systems Administrator http://jdc.koitsu.org/ |
| Mountain View, CA, US |
| Making life hard for others since 1977. PGP 4BD6C0CB |
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