Re: Bug: Buffer cache is not scan resistant

Mark Kirkwood <markir@paradise.net.nz> writes:
> Shared Buffers  Elapsed  IO rate (from vmstat)
> --------------  -------  ---------------------
> 400MB           101 s    122 MB/s
> 2MB             100 s
> 1MB              97 s
> 768KB            93 s
> 512KB            86 s
> 256KB            77 s
> 128KB            74 s    166 MB/s

Hm, that seems to blow the "it's an L2 cache effect" theory out of the
water.  If it were a cache effect then there should be a performance
cliff at the point where the cache size is exceeded.  I see no such
cliff, in fact the middle part of the curve is darn near a straight
line on a log scale ...

So I'm back to asking what we're really measuring here.  Buffer manager
inefficiency of some sort, but what?  Have you tried oprofile?
        regards, tom lane