On Sat, Apr 19, 2014 at 1:07 AM, Bruce Momjian <bruce@momjian.us> wrote:
On Fri, Apr 18, 2014 at 04:46:31PM +0530, Atri Sharma wrote: > This can be changed by introducing an ageing factor that sees how much time the > current buffer has spend in shared buffers. If the time that the buffer has > spent is large enough (relatively) and it is not hot currently, that means it > has had its chance and can be evicted. This shall save the new page (3) from > being evicted since it's time in shared buffers shall not be high enough to > mandate eviction and it shall be given more chances. > > Since gettimeofday() is an expensive call and hence cannot be done in the tight > loop, we can count the number of clocksweeps the current buffer has seen > (rather, survived). This shall give us a rough idea of the estimate of the > relative age of the buffer.
Counting clock sweeps is an intersting idea. I think one concern was tracking hot buffers in cases where there is no memory pressure, and hence the clock sweep isn't running --- I am not sure how this would help in that case.
I feel that if there is no memory pressure, frankly it doesnt matter much about what gets out and what not. The case I am specifically targeting is when the clocksweep gets to move about a lot i.e. high memory pressure workloads. Of course, I may be totally wrong here.
One thing that I discussed with Merlin offline and am now concerned about is how will the actual eviction work. We cannot traverse the entire list and then find all the buffers with refcount 0 and then do another traversal to find the oldest one.