On Fri, Apr 03, 2009 at 06:52:26PM -0400, Tom Lane wrote: - Greg Smith <gsmith@xxxxxxxxxxxxx> writes: - > pgbench is extremely bad at simulating large numbers of clients. The - > pgbench client operates as a single thread that handles both parsing the - > input files, sending things to clients, and processing their responses. - > It's very easy to end up in a situation where that bottlenecks at the - > pgbench client long before getting to 400 concurrent connections. - - Yeah, good point. hmmm ok, I didn't realize that pgbouncer wasn't threaded. I've got a Plan B that doesn't use pgbouncer that i'll try. - > That said, if you're in the hundreds of transactions per second range that - > probably isn't biting you yet. I've seen it more once you get around - > 5000+ things per second going on. - - However, I don't think anyone else has been pgbench'ing transactions - where client-side libpq has to absorb (and then discard) a megabyte of - data per xact. I wouldn't be surprised that that eats enough CPU to - make it an issue. David, did you pay any attention to how busy the - pgbench process was? I can run it again and have a look, no problem. - Another thing that strikes me as a bit questionable is that your stated - requirements involve being able to pump 400MB/sec from the database - server to your various client machines (presumably those 400 people - aren't running their client apps directly on the DB server). What's the - network fabric going to be, again? Gigabit Ethernet won't cut it... Yes, sorry I'm not trying to be confusing but i didn't want to bog everyone down with a ton of details. 400 concurrent users doesn't mean that they're pulling 1.5 megs / second every second. Just that they could potentially pull 1.5 megs at any one second. most likely there is a 6 (minimum) to 45 second (average) gap between each individual user's pull. My plan B above emulates that, but i was using pgbouncer to try to emulate "worst case" scenario. - The point I was trying to make is that it's the disk subsystem, not - the CPU, that is going to make or break you. Makes sense, I definitely want to avoid I/Os. On Fri, Apr 03, 2009 at 05:51:50PM -0400, Greg Smith wrote: - Wrapping a SELECT in a BEGIN/END block is unnecessary, and it will - significantly slow down things for two reason: the transactions overhead - and the time pgbench is spending parsing/submitting those additional - lines. Your script should be two lines long, the \setrandom one and the - SELECT. - Oh perfect, I can try that too. thanks - The thing that's really missing from your comments so far is the cold - vs. hot cache issue: at the point when you're running pgbench, is a lot I'm testing with a cold cache because most likely the way the items are spead out, of those 400 users only a few at a time might access similar items. - Wait until Monday, I'm announcing some pgbench tools at PG East this - weekend that will take care of all this as well as things like - graphing. It pushes all the info pgbench returns, including the latency - information, into a database and generates a big stack of derived reports. - I'd rather see you help improve that than reinvent this particular wheel. Ah very cool, wish i could go (but i'm on the west coast). Thanks again guys. Dave Kerr -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
