Re: Seq scans status update

Alvaro Herrera wrote:
> Tom Lane wrote:
>> Gregory Stark <stark@enterprisedb.com> writes:
>>> Is there a reason UnpinBuffer has to be the one to increment the usage count
>>> anyways? Why can't ReadBuffer handle incrementing the count and just trust
>>> that it won't be decremented until the buffer is unpinned anyways?
>> That's a good question.  I think the idea was that if we hold a buffer
>> pinned for awhile (long enough that the bgwriter's clock sweep passes
>> over it one or more times), we want the usage count decrementing to
>> start when we release the pin, not when we acquire it.  But maybe that
>> could be fixed if the clock sweep doesn't touch the usage_count of a
>> pinned buffer.  Which in fact it may not do already --- didn't look.
>
> It does -- in BgBufferSync the "all" scan calls SyncOneBuffer with
> skip_pinned=false.  The "lru" scan does skip pinned buffers.

You're looking at the wrong place. StrategyGetBuffer drives the clock
sweep, and it always decreases the usage_count, IOW it doesn't skip
pinned buffers. SyncOneBuffer and BgBufferSync don't decrease the
usage_count in any case.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com