On Dec 25, 2009, at 9:57 AM, Saso Kiselkov wrote:
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I've started porting a video streaming application to opensolaris on
ZFS, and am hitting some pretty weird performance issues. The thing
I'm
trying to do is run 77 concurrent video capture processes (roughly
430Mbit/s in total) all writing into separate files on a 12TB J4200
storage array. The disks in the array are arranged into a single
RAID-0
ZFS volume (though I've tried different RAID levels, none helped). CPU
performance is not an issue (barely hitting 35% utilization on a
single
CPU quad-core X2250). I/O bottlenecks can also be ruled out, since the
storage array's sequential write performance is around 600MB/s.
The problem is the bursty behavior of ZFS writes. All the capture
processes do, in essence is poll() on a socket and then read() and
write() any available data from it to a file.
There have been some changes recently, including one in b130 that
might apply to this workload. What version of the OS are you running?
If not b130, try b130.
-- richard
The poll() call is done
with a timeout of 250ms, expecting that if no data arrives within 0.25
seconds, the input is dead and recording stops (I tried increasing
this
value, but the problem still arises, although not as frequently). When
ZFS decides that it wants to commit a transaction group to disk (every
30 seconds), the system stalls for a short amount of time and
depending
on the number capture of processes currently running, the poll() call
(which usually blocks for 1-2ms), takes on the order of hundreds of
ms,
sometimes even longer. I figured that I might be able to resolve
this by
lowering the txg timeout to something like 1-2 seconds (I need ZFS to
write as soon as data arrives, since it will likely never be
overwritten), but I couldn't find any tunable parameter for it
anywhere
on the net. On FreeBSD, I think this can be done via the
vfs.zfs.txg_timeout sysctl. A glimpse into the source at
http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/fs/zfs/txg.c
on line 40 made me worry that somebody maybe hard-coded this value
into
the kernel, in which case I'd be pretty much screwed in opensolaris.
Any help would be greatly appreciated.
Regards,
- --
Saso
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