On Sat, Aug 13, 2011 at 12:18 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
>> Yeah. Also, to my mind this is only a fix that will be used in 9.0 and
>> 9.1 --- now that it's occurred to me that we could use tuple xmin/xmax
>> to invalidate catcaches instead of recording individual TIDs, I'm
>> excited about doing that instead for 9.2 and beyond. I believe that
>> that could result in a significant reduction in sinval traffic, which
>> would be a considerable performance win.
>
> On closer inspection this idea doesn't seem workable. I was imagining
> that after a transaction commits, we could find obsoleted catcache
> entries by looking for tuples with xmax equal to the transaction's XID.
> But a catcache entry made before the transaction had done the update
> wouldn't contain the correct xmax, so we'd fail to remove it. The only
> apparent way to fix that would be to go out to disk and consult the
> current on-disk xmax, which would hardly be any cheaper than just
> dropping the cache entry and then reloading it when/if needed.
>
> All is not entirely lost, however: there's still some possible
> performance benefit to be gained here, if we go to the scheme of
> identifying victim catcache entries by hashvalue only. Currently,
> each heap_update in a cached catalog has to issue two sinval messages
> (per cache!): one against the old TID and one against the new TID.
> We'd be able to reduce that to one message in the common case where the
> hashvalue remains the same because the cache key columns didn't change.
Cool.
> Another thing we could consider doing, if one-in-2^32 hash collisions
> seems too high, is widening the hash values to 64 bits. I'm not
> terribly excited about that, because a lot of the caches are on OID
> columns for which there'd be zero benefit.
Yeah, and I can't get excited about the increased memory usage, either.
--
Robert Haas
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