Possible ZFS livelock or SCHED_ULE bug ?

[Arnaud Houdelette]

Hi all !
I got a UniProcessor AMD64 box, with 512 MB ram with 2 ZFS pools as a 
home-NAS.

I got some IO issues since I moved from 7.2 to 8.0.
With a GENERIC kernel (or a stripped down one),  during high IO activity 
(as a make buildword can cause), I encounter random hangs or deadlocks.
top show system CPU usage at 99%, the most CPU using process being 
[zfskern] ( {txg_thread_enter} if I switch to thread view).
The box still respond to ping. Current processes can still run, but I 
can't run new ones.
Sometimes, I can return to normal by Ctrl-C-ing the buildworld (or other 
operation), sometimes I can't, I got to reboot the box.

The Issue seemed to become less frequent with 8.0-stable instead of 
8.0-RELEASE, but still present (I get approximately 75% chance of hang 
with a buildworld).
I got the hang with Prefetch enabled or disabled. Idem for ZIL.

I tried to enable kernel dumps, but the box hangs saving the dump (root 
is on ZFS) or when starting kdbg on it.
I recompiled kernel with SCHED_4BSD, and it seems I can't reproduce the 
hang.

What do you think ?
Did I misconfigured something ?


cat /boot/loader.conf
zfs_load="YES"
vfs.root.mountfrom="zfs:unsafe/root"
vm.kmem_size="512M"
vm.kmem_size_max="512M"
vfs.zfs.arc_max="100M"
vfs.zfs.vdev.cache.size="10M"
vfs.zfs.prefetch_disable="0"
vfs.zfs.zil_disable="1"


[carenath] ~> zpool status
  pool: tank
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            ad6p1   ONLINE       0     0     0
            ad10p1  ONLINE       0     0     0
            ad8p1   ONLINE       0     0     0
            ad4p1   ONLINE       0     0     0

errors: No known data errors

  pool: unsafe
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        unsafe      ONLINE       0     0     0
          ad0p3     ONLINE       0     0     0

errors: No known data errors