Re: Hung task - sync - 2.6.33-rc7 w/md6 multicore rebuild in process

[Michael Breuer <mbreuer@majjas.com>]

On 2/18/2010 11:02 PM, Dave Chinner wrote:
> On Thu, Feb 18, 2010 at 09:31:41PM -0500, Michael Breuer wrote:
>    
>> On 2/18/2010 8:43 PM, Dave Chinner wrote:
>>      
>>>
>>> This is probably where the barrier IOs are coming from.  With a RAID
>>> resync going on (so all IO is going to be slow to begin with) and
>>> writeback is causing barriers to be issued (which are really slow on
>>> software RAID5/6), having sync take so long is not out of the
>>> question if you have lots of dirty inodes to write back. A kernel
>>> compile will generate lots of dirty inodes.
>>>
>>> Even taking the barrier IOs out of the question, I've seen reports
>>> of sync or unmount taking over 10 hours to complete on software
>>> RAID5 because there were hundreds of thousands of dirty inodes to
>>> write back and each inode being written back caused a synchronous
>>> RAID5 RMW cycle to occur. Hence writeback could only clean 50
>>> inodes/sec because as soon as RMW cycles RAID5/6 devices start
>>> they go slower than single spindle devices.  This sounds very
>>> similar to what you are seeing here,
>>>
>>> i.e. The reports don't indicate to me that there is a bug in the
>>> writeback code, just your disk subsystem has very, very low
>>> throughput in these conditions....
>>>        
>> Probably true... and the system does recover. The only thing I'd point
>> out is that the subsystem isn't (or perhaps shouldn't) be this sluggish.
>> I hypothesize that the low throughput under these condition is a result
>> of:
>> 1) multicore raid support (pushing the resync at higher rates)
>>      
> Possibly, though barrier support for RAID5/6 is shiny new as well.
>
>    
>> 2) time spent in fs cache reclaim. The sync slowdown only occurs when fs
>> cache is in heavy (10Gb) use.
>>      
> Not surprising ;)
>
>    
>> I actually could not recreate the issue until I did a grep -R foo /usr/
>>      
>>> /dev/null to force high fs cache utilization. For what it's worth, two
>>>        
>> kernel rebuilds (many dirty inodes) and then a sync with about 12Mb
>> dirty (/proc/meminfo) didn't cause an issue. The issue only happens when
>> fs cache is heavily used. I also never saw this before enabling
>> multicore raid.
>>      
> "grep -R foo /usr/" will dirty every inode that touchs (atime) and
> they have to be written back out. That's almost certainly creating
> more dirty inodes than a kernel build - there are about 400,000
> inodes under /usr on my system. That would be enough to trigger very
> long sync times if inode writeback is slow.
>
> Cheers,
>
> Dave.
>    

My filesystems are mounted relatime. Just confirmed that dirty pages 
doesn't climb all that much with the grep -R foo /usr > /dev/null. The 
only apparant impact is to fs cache.