On 1/14/14, 11:30 AM, Jeff Janes wrote:
I think the "reclaim this page if you need memory but leave it resident if there is no memory 
pressure" hint would be more useful for temporary working files than for what was being 
discussed above (shared buffers).  When I do work that needs large temporary files, I often see 
physical write IO spike but physical read IO does not.  I interpret that to mean that the temporary 
data is being written to disk to satisfy either dirty_expire_centisecs or dirty_*bytes, but the 
data remains in the FS cache and so disk reads are not needed to satisfy it.  So a hint that says 
"this file will never be fsynced so please ignore dirty_*bytes and dirty_expire_centisecs.  I 
will need it again relatively soon (but not after a reboot), but will do so mostly sequentially, so 
please don't evict this without need, but if you do need to then it is a good candidate" would 
be good.

I also frequently see this, and it has an even larger impact if pgsql_tmp is on 
the same filesystem as WAL. Which *theoretically* shouldn't matter with a BBU 
controller, except that when the kernel suddenly decides your *temporary* data 
needs to hit the media you're screwed.

Though, it also occurs to me... perhaps it would be better for us to simply map 
temp objects to memory and let the kernel swap them out if needed...
Jim C. Nasby, Data Architect                       j...@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

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