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On 22-Mar-2013, at 11:28, Amit Kapila <amit.kap...@huawei.com> wrote: > On Friday, March 22, 2013 10:22 AM Atri Sharma wrote: >> Hello all, >> >> Sorry if this is a naive question. >> >> I was going through Greg Smith's slides on buffer >> cache(http://www.westnet.com/~gsmith/content/postgresql/InsideBufferCac >> he.pdf). >> When going through the page replacement algorithm that we use i.e. >> clocksweep algorithm, I felt a potential problem in our current >> system. >> >> Specifically, when a new entry is allocated in the buffer, it's >> USAGE_COUNT is set to 1. On each sweep of the algorithm, the >> USAGE_COUNT is decremented and an entry whose USAGE_COUNT becomes >> zero is replaced. > > Yes, it is replaced but in the next clock sweep pass, not immediately after > making 0. > So till the time of next pass if nobody accesses the buffer and all other > buffers have higher count, it can be replaced. > Also the buffer, it has returned for which the usage count becomes 1, it > will come to reduce the usage count only in next pass. > So in whole, I think it needs 2 passes for a freshly returned buffer to be > re-used incase no one uses it again. > > With Regards, > Amit Kapila. > Hmm,so in the second pass,it gets replaced,right? I think that if the initialization of USAGE_COUNT starts at the maximum allowed value instead of one, we can have a better solution to this problem. Another,more complex solution could be to introduce an ageing factor as well. Regards, Atri -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
