So I have this dual 16-core Opteron Dell R715 with 128G of RAM attached
to a SuperMicro disk enclosure with 45 2TB Toshiba SAS drives (via two
LSI 9200 controllers and MPxIO) running OpenIndiana 151a4 and I'm
occasionally seeing a storm of xcalls on one of the 32 VCPUs (>100000
xcalls a second). The machine is pretty much idle, only receiving a
bunch of multicast video streams and dumping them to the drives (at a
rate of ~40MB/s). At an interval of roughly 1-2 minutes I get a storm of
xcalls that completely eat one of the CPUs, so the mpstat line for the
CPU looks like:
CPU minf mjf xcal intr ithr csw icsw migr smtx srw syscl usr sys wt idl
31 0 0 102191 1 0 0 0 0 0 0 0 0 100
0 0
100% busy in the system processing cross-calls. When I tried dtracing
this issue, I found that this is the most likely culprit:
dtrace -n 'sysinfo:::xcalls {@[stack()]=count();}'
unix`xc_call+0x46
unix`hat_tlb_inval+0x283
unix`x86pte_inval+0xaa
unix`hat_pte_unmap+0xed
unix`hat_unload_callback+0x193
unix`hat_unload+0x41
unix`segkmem_free_vn+0x6f
unix`segkmem_zio_free+0x27
genunix`vmem_xfree+0x104
genunix`vmem_free+0x29
genunix`kmem_slab_destroy+0x87
genunix`kmem_slab_free+0x2bb
genunix`kmem_magazine_destroy+0x39a
genunix`kmem_depot_ws_reap+0x66
genunix`taskq_thread+0x285
unix`thread_start+0x8
3221701
This happens in the sched (pid 0) process. My fsstat one looks like this:
# fsstat /content 1
new name name attr attr lookup rddir read read write write
file remov chng get set ops ops ops bytes ops bytes
0 0 0 664 0 952 0 0 0 664 38.0M /content
0 0 0 658 0 935 0 0 0 656 38.6M /content
0 0 0 660 0 946 0 0 0 659 37.8M /content
0 0 0 677 0 969 0 0 0 676 38.5M /content
What's even more puzzling is that this happens apparently entirely
because of some factor other than userland, since I see no changes to
CPU usage of processes in prstat(1M) when this xcall storm happens, only
an increase of loadavg of +1.00 (the busy CPU).
I Googled and found that
http://mail.opensolaris.org/pipermail/dtrace-discuss/2009-September/008107.html
seems to have been an issue identical to mine, however, it remains
unresolved at that time and it worries me about putting this kind of
machine into production use.
Could some ZFS guru please tell me what's going on in segkmem_zio_free?
When I disable the writers to the /content filesystem, this issue goes
away, so it has obviously something to do with disk IO. Thanks!
Cheers,
--
Saso
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