Re: What's the NFS OOM problem?

[Roger Heflin <rheflin@atipa.com>]

Neil Brown wrote:
> On Tuesday August 15, rheflin@atipa.com wrote:
>> I have noticed on SLES kernels that when the dirty_*ratios turned down it
>> still uses alot more memory than it should work writeback buffers, it makes
>> me think that with the default setting of 40% that it for some reason
>> may be using all of memory and deadlocking.   It does not seem like an
>> NFS only issue, as I believe I have duplicated it with a fast lock
>> setup.
> 
> We seem to have a little patch in SuSE kernels that might be making
> the problem worse .... though I presume it was introduced for a
> reason.  I haven't managed to track what that reason was yet.
> 
> What is "a fast lock setup"??  I don't understand.
> 
> NeilBrown
> 

I am not sure what I ment, I may have ment a fast disk setup, and
thought or typed the wrong thing.  The machine I duplicated it with
had disks that would sustain 175MB/second (3 striped), 4cpus with local
ram of 32GB.   The 2 cpu/4GB/100MB/second machine does not seem
to have the issue.   Both machines are opterons, I believe I duplicated
it under SP2, I know I duplicated it SP3 and one of the
post-SP3 kernels.   It did not occur under SP1.

Turning down the dirty*ratios seems to make it go away.   When I
get a chance I will retest on SP2 and see if it happens there.

I do know (and this may be related) that if on a 32GB machine I
pagelock a large portion of ram (say 28GB) that machine will deadlock
under high IO.  The basic symptoms are similar to the writeback
issue the machine responds to ping/sysrq, but logins fail, and any
new process creation fails.

                                  Roger