[linux-mm-cc] swap-device write failure under low memory

Nitin Gupta ngupta at vflare.org
Sat Jul 10 04:27:37 EDT 2010


Hi,

On 07/07/2010 02:00 PM, Uma shankar wrote:

>                 Occasionally,  xvMalloc  will  try to grow the pool.
> This memory allocation can fail  under low memory.
> 
> This will be informed to the kernel as  a "device write" failure.
> The page which was being written will not be reclaimed, but
> the kernel will  continue to  try swap out of other pages ( as kernel
> thinks that swap has free space. )
> 
> Wont this lead  to the reclaim code ( kswapd  or the direct reclaim
> path )  hogging  the processor for some time before  OOM is
> finally announced ?
> 
> And what if a NOFAIL allocation attempt results in this ?
> 
> Have you analysed this scenario ?
> 


If you ever get too many write failures, its an indication that you
have oversized ramzswap device. From what I have observed, swap write
failure quickly lead to system hang. In ideal case, ramzswap should
be able to dynamically resize based on cache hit-rate, system memory
pressure etc., but all this is not yet done. So, for now, best would
be to experiment a bit and get some idea of ramzswap disksize that helps
your workload while still not getting into messy conditions like OOM,
swap write failures.

Thanks,
Nitin


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