Re: [PATCH] Deadlock during heavy write activity to userspace NFS server on local NFS mount

[Avi Kivity <avi@exanet.com>]

Pavel Machek wrote:

>>On heavy write activity, allocators wait synchronously for kswapd to
>>free some memory. But if kswapd is freeing memory via a userspace NFS
>>server, that server could be waiting for kswapd, and the system seizes
>>instantly.
>>
>>This patch (against RHEL 2.4.21-15EL, but should apply either 
>>literally
>>or conceptually to other kernels) allows a process to declare itself 
>>as
>>kswapd's little helper, and thus will not have to wait on kswapd.
>>    
>>
>
>Ok, but what if its memory runs out, anyway?
>
>  
>
Tough. What if kswapd's memory runs out?

A more complete solution would be to assign memory reserve levels below 
which a process starts allocating synchronously. For example, normal 
processes must have >20MB to make forward progress, kswapd wants >15MB 
and the NFS server needs >10MB. Some way would be needed to express the 
dependencies.

I think more and more people will hit this problem as filesystems become 
more complex due to clustering, and migrate to userspace where it can be 
more easily managed.

Avi