Re: [HACKERS] Re: [COMMITTERS] pgsql: Revert commit_delay change; just add comment that we don't hav
On 08/15/2012 11:41 AM, Peter Geoghegan wrote: I know that someone is going to point out that in some particularly benchmark, they can get another relatively modest increase in throughput (perhaps 2%-3%) by splitting the difference between two adjoining millisecond integer values. In that scenario, I'd be tempted to point out that that increase is quite unlikely to carry over to real-world benefits, because the setting is then right on the cusp of where increasing commit_delay stops helping throughput and starts hurting it. You guessed right on that. I just responded to your survey over on pgsql-performance with two cases where older versions found optimal performance with commit_delay in the =10 usec range. Those are all in the BBWC case that I don't think you've been testing much of yet. I recall Jignesh Shah reported his seeing that was from slightly better chunking of writes to disk, with a small but measurable drop in disk I/O operations (such as IOPS) relative to TPS. The average throughput was no different; the number of *operations* was smaller though. Less 8K I/O requests, more 16K+ ones. Like a lot of these situations, adding some latency to every transactions can make them batch better. And that can unexpectedly boost throughput enough that net latency is actually faster. It's similar to how adding input queue latency with a pooler, limiting active connections, can actually make latency better by increasing efficiency. On higher-end storage you can reach a point where IOPS gets high enough that the per-operation overhead becomes a problem, on top of the usual is there enough write throughput? question. I suspect this situation might even be more common now, given IOPS issues like this are commonly highlighted when people do SSD reviews. I still don't know that it's a widely popular situation. But this particular use case has been one of the more persistent ones arguing to keep the parameter around until now. Making sub-microsecond resolution on the parameter go away would effectively trash it just when it might get even more useful than before. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] Re: [COMMITTERS] pgsql: Revert commit_delay change; just add comment that we don't hav
On 15 August 2012 05:15, Tom Lane t...@sss.pgh.pa.us wrote: Peter Geoghegan pe...@2ndquadrant.com writes: On 14 August 2012 21:26, Bruce Momjian br...@momjian.us wrote: Revert commit_delay change; just add comment that we don't have a microsecond specification. I think that if we eventually decide to change the name of commit_delay for 9.3 (you previously suggested that that might be revisited), it will be reasonable to have the new GUC in units of milliseconds. Well, the reason why it's like that at all is the thought that values of less than 1 millisecond might be useful. Are we prepared to suppose that that is not and never will be true? I think that the need for sub-millisecond granularity had more to do with historic quirks of our implementation. Slight tweaks accidentally greatly improved throughput, if only for the synthetic benchmark in question. I personally have not seen any need for a sub-millisecond granularity when experimenting with the setting on 9.3-devel. However, I am not sure that sub-millisecond granularity could never be of any use, in squeezing the last small increase in throughput made possible by commit_delay. Importantly, feedback as the GUC is tuned is far more predictable than it was with the prior implementation, so this does seem quite unimportant. Why does commit_delay have to be an integer? Can't we devise a way of manipulating it in units of milliseconds, but have the internal representation be a double, as with pg_stat_statements' total_time column? That would allow very fine tuning of the delay. As I've outlined, I'm not sure that it's worth supporting such fine-tuning with the new implementation. -- Peter Geoghegan http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] Re: [COMMITTERS] pgsql: Revert commit_delay change; just add comment that we don't hav
On 15 August 2012 14:39, Tom Lane t...@sss.pgh.pa.us wrote: If you wanted to re-implement all the guc.c logic for supporting unit-ified values such that it would also work with floats, we could do that. It seems like way more mechanism than the problem is worth however. Fair enough. I'm not quite comfortable recommending a switch to milliseconds if that implies a loss of sub-millisecond granularity. I know that someone is going to point out that in some particularly benchmark, they can get another relatively modest increase in throughput (perhaps 2%-3%) by splitting the difference between two adjoining millisecond integer values. In that scenario, I'd be tempted to point out that that increase is quite unlikely to carry over to real-world benefits, because the setting is then right on the cusp of where increasing commit_delay stops helping throughput and starts hurting it. The improvement is likely to get lost in the noise in the context of a real-world application, where for example the actually cost of an fsync is more variable. I'm just not sure that that's the right attitude. -- Peter Geoghegan http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
