On Tue, Feb 10, 2004 at 01:20:32PM -0700, Ed L. wrote: > On Friday January 30 2004 6:06, Martijn van Oosterhout wrote: > > On Fri, Jan 30, 2004 at 03:19:56PM -0700, Ed L. wrote: > > > > > > I'm also curious about the relationship of DB shared buffer cache to > > > the linux/hpux kernel caches. ... > > > > Whenever the database needs a block not in memory it get loaded and ends > > up in both the OS cache and the DB cache. The difference between getting > > a block out of DB cache and OS cache is very, very small compared to > > loading off disk. > > So the OS cache and the DB cache may well have duplicate copies of the data? > With OS cache being more susceptible to preemption from other processes?
Will have. Any time postgresql does a read() the kernel is copying a buffer from it's cache to postgresql's internal cache. The kernel cache is more susceptable to reuse by other processes, but it's still double. > > Finally, at least on Linux, the shared memory postgres uses for cache can > > also be swapped out making it very difficult to determine the correct > > value. > > What would cause DB shared memory to be swapped out in linux? I thought it > was all pre-allocated at pgsql startup. Is this the "well its not really > pre-allocated but rather supplied on demand" story? I think I saw some > sort of kernel parameter controlling a similar (same?) policy... Memory pressure. Same rules apply as for everything else I beleive. If you allocate a huge amount to shared memory, fill it up and then don't use it for a while while the rest of the system needs the memory, it'll get swapped out. I generally give Postgresql about 64-128MB of shared memory, which covers all of the system tables and the most commonly used small tables. The rest of the memory (this is a 1GB machine) I leave for the kernel to manage for the very large tables. I think giving postgresql half your memory is couterproductive. -- Martijn van Oosterhout <[EMAIL PROTECTED]> http://svana.org/kleptog/ > (... have gone from d-i being barely usable even by its developers > anywhere, to being about 20% done. Sweet. And the last 80% usually takes > 20% of the time, too, right?) -- Anthony Towns, debian-devel-announce
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