Re: ~500 megs cached yet 2.6.5 goes into swap hell

["Brett E." <brettspamacct@fastclick.com>]

Andrew Morton wrote:

> "Brett E." <brettspamacct@fastclick.com> wrote:
> 
>>>I see no swapout from the info which you sent.
>>
>> pgpgout/s gives the total number of blocks paged out to disk per second, 
>> it peaks at 13,000 and hovers around 3,000 per the attachment.
> 
> 
> Nope.  pgpgout is simply writes to disk, of all types.
That is what is confusing me.. From the sar man page:

pgpgin/s
     Total number of kilobytes the system paged in from disk per second.

pgpgout/s
     Total number of kilobytes the system paged out to disk per second.



> 
> swapout is accounted for under pswpout and your vmstat trace shows a little
> bit of (healthy) swapout with swappiness=100 and negligible swapout with
> swappiness=0.  In both cases, negligible swapin.  That's all just fine.
> 
> 
>> Swapping out is good, but when that's coupled with swapping in as is the 
>> case on my side, it creates a thrashing situation where we swap out to 
>> disk pages which are being used, we then immediately swap those pages 
>> back in, etc etc..
> 
> 
> Look at your "si" column in vmstat.  It's practically all zeroes.
> 
> 
>> The usage pattern by the way is on a server which continuously hits a 
>> database and reads files so I don't know what "swappiness" should be set 
>> to exactly.  Every hour or so it wants to untar tarballs and by then the 
>> cache is large. From here, the system swaps in and out more while cache 
>> decreases. Basically, it should do what I believe Solaris does... simply 
>> reclaim cache and not swap.  Capping cache would be good too but the 
>> best solution IMO is to simply reclaim the cache on an as-needed basis 
>> before thinking about swapping.
> 
> 
> swappiness=100: swaps a lot.  swappiness=0: doesn't swap much.
> 
> With a funny workload like that you might choose to set swappiness to 0
> just around the hourly tar operation, but as the machine seems to not be
> swapping there doesn't seem to be a need.

Yeah, it wouldn't help if paging isn't the problem. I'd like more 
clarificaton on sar before I throw out paging being the culprit.