Sorry for top-posting - the iphone mail client sucks.
I think what's happening is that the sytem is seeing that some pages
of shared memory haven't been used recently and because there's more
shared memory than filesystem cache less recently than the filesystem
cache pages. So it pages out the shared memory. This is really awful
because we use a kind of lru algorithm for shared memory so the pages
that it's paging out are precisely the pges likely to be used soon.
I wonder if we should try to mlock shared buffers.
--
Greg
On 4 May 2009, at 10:10, Laurent Laborde <kerdezixe@xxxxxxxxx> wrote:
Friendly greetings !
I found something "odd" (something that i can't explain) this weekend.
An octocore server with 32GB of ram, running postgresql 8.3.6
Running only postgresql, slony-I and pgbouncer.
Just for testing purpose, i tried a setting with 26GB of
shared_buffer.
I quickly noticed that the performances wasn't very good and the
server started to swap slowly but surely.
(but still up to 2000query/second as reported by pgfouine)
It used all the 2GB of swap.
I removed the server from production, added 10GB of swap and left it
for the weekend with only slony and postgresql up to keep it in sync
with the master database.
This morning i found that the whole 12GB of swap were used :
Mem: 32892008k total, 32714728k used, 177280k free, 70872k
buffers
Swap: 12582896k total, 12531812k used, 51084k free, 27047696k
cached
# cat /proc/meminfo
MemTotal: 32892008 kB
MemFree: 171140 kB
Buffers: 70852 kB
Cached: 27065208 kB
SwapCached: 4752492 kB
Active: 24362168 kB
Inactive: 7806884 kB
HighTotal: 0 kB
HighFree: 0 kB
LowTotal: 32892008 kB
LowFree: 171140 kB
SwapTotal: 12582896 kB
SwapFree: 53064 kB
Dirty: 122636 kB
Writeback: 0 kB
AnonPages: 280336 kB
Mapped: 14118588 kB
Slab: 224632 kB
PageTables: 235120 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
CommitLimit: 29028900 kB
Committed_AS: 28730620 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 12916 kB
VmallocChunk: 34359725307 kB
While i understand that a very high shared_buffer wasn't a good idea,
i don't understand this behaviour.
Any tought ?
I tried this setup because having 2 level of data caching doesn't make
sense to me. (1 in OS filesystem cache and 1 in shm (shared_buffer)).
I'd love to understand what's happening here ! Thank you :)
--
F4FQM
Kerunix Flan
Laurent Laborde
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