Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
On Wed, Feb 22, 2012 at 6:53 AM, Greg Smith g...@2ndquadrant.com wrote: A look back on this now that I'm done with it does raise one large question though. I added some examples of how to measure timing overhead using psql. While I like the broken down timing data that this utility provides, I'm not sure whether it's worth adding a contrib module just to get it now though. Extension that's packaged on something like PGXN and easy to obtain? Absolutely--but maybe that's a developer only level thing. Maybe the only code worth distributing is the little SQL example of how to measure the overhead, along with some reference good/bad numbers. That plus the intro to timer trivia could turn this into a documentation section only, no code change. I've dreamed of running something like this on every system in the build farm. Even if that's a valuable exercise, even then it may only be worth doing once, then reverting. I had similar concerns, but decided to go ahead and commit this. It doesn't relate particularly closely to the buffer I/O timings patch, but I think it's worth having. We clearly need something that is integrated with the PostgreSQL sources, because there is more than one way to access timers, and this code will measure the overhead of doing what PostgreSQL does, rather than the overhead of reading times in some other way. Now, that doesn't preclude shipping this on PGXN, but we've certainly got other things in contrib that are clearly a lot less useful than this, and we've discussed before the folly of shipping a database product without a full set of diagnostic tools. Since this tool was developed to answer questions about whether certain PostgreSQL options are safe to enable or whether they'll have too much overhead, I think that's a sufficient reason to include it in contrib, especially because you took the time to write some very good documentation for it. I wonder whether we should perhaps move some of this discussion of clock sources into the main documentation somewhere, since it's not specifically related to this utility, but I didn't try to do that for the moment and just left it as you had it. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
Greg Smith wrote: Anyway, the patch does now includes several examples and a short primer on PC clock hardware, to help guide what good results look like and why they've been impossible to obtain in the past. That's a bit Linux-centric, but the hardware described covers almost all systems using Intel or AMD processors. Only difference with most other operating systems is how aggressively they have adopted newer timer hardware. At least this gives a way to measure all of them. N.B.: Windows has at least two clock APIs, timeGetTime and QueryPerformanceCounters (and probably more, these days). They rely on different hardware clocks, and can get out of sync with each other; meanwhile, QueryPerformanceCounters can get out of sync with itself on (older?) multi-CPU boards. So if you're doing high-res timing, it's good to make sure you aren't relying on two different clocks in different places... I ran into this with MIDI drivers years ago; and wrote a doc: http://www.ultimatemetal.com/forum/equipment/315910-midi-latency-problem-nuendo.html#post6315034 and a clock-testing utility: https://github.com/jaylevitt/miditime -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
On 02/22/2012 11:10 AM, Jay Levitt wrote: N.B.: Windows has at least two clock APIs, timeGetTime and QueryPerformanceCounters (and probably more, these days). They rely on different hardware clocks, and can get out of sync with each other; meanwhile, QueryPerformanceCounters can get out of sync with itself on (older?) multi-CPU boards. The PostgreSQL wrapper in src/include/portability/instr_time.h uses QueryPerformanceCounter and QueryPerformanceFrequency in a way that the result can be used similarly to how deltas in UNIX dates are returned. As far as I've been able to tell, there aren't any issues unique to Windows there. Multiple cores can have their TSC results get out of sync on Windows for the same reason they do on Linux systems, and there's also the same frequency/temperature issues. Newer versions of Windows can use TSC, older versions only use HPET or ACPI on older versions, and there's some ability to force bad TSC units to use ACPI instead: http://blogs.msdn.com/b/psssql/archive/2010/08/18/how-it-works-timer-outputs-in-sql-server-2008-r2-invariant-tsc.aspx http://blogs.msdn.com/b/psssql/archive/2010/08/18/how-it-works-timer-outputs-in-sql-server-2008-r2-invariant-tsc.aspx There is a lot of questionable behavior if you try to use the better timers in Windows XP; check out the obnoxious foot note about XP SP3 at http://en.wikipedia.org/wiki/High_Precision_Event_Timer for example. Since there's little you can do about--it's not even possible to figure out which timer is being used easily--I didn't see any easy ways to document what Windows does here, in a way that helped anyone. For the most part, you get the best timer Windows has a driver for with QueryPerformanceCounter, and that's what the PostgreSQL code uses. The information I gave about how newer systems can have more accurate timing is still accurate. Maybe it would be useful to add something pointing out that newer Windows versions tend to support faster timers, too. That's something I assume people would guess from what I already wrote, it may be worth stating explicitly though. -- 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
Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
On Wed, Feb 22, 2012 at 18:44, Greg Smith g...@2ndquadrant.com wrote: As far as I've been able to tell, there aren't any issues unique to Windows there. Multiple cores can have their TSC results get out of sync on Windows for the same reason they do on Linux systems, and there's also the same frequency/temperature issues. Not on recent Linux kernel versions. Linux automatically detects when the TSC is unstable (due to power management or out-of-sync cores/sockets) and automatically falls back to the more expensive HPET or ACPI methods. e.g: % dmesg |grep -i tsc [0.00] Fast TSC calibration using PIT [0.164075] checking TSC synchronization [CPU#0 - CPU#1]: passed. [0.197062] Switching to clocksource tsc [0.260960] Marking TSC unstable due to TSC halts in idle Whether these tests cover 100% of the possible conditions, and whether the detection has race conditions or not, I don't know. Regards, Marti -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
On 02/22/2012 12:25 PM, Marti Raudsepp wrote: On Wed, Feb 22, 2012 at 18:44, Greg Smithg...@2ndquadrant.com wrote: As far as I've been able to tell, there aren't any issues unique to Windows there. Multiple cores can have their TSC results get out of sync on Windows for the same reason they do on Linux systems, and there's also the same frequency/temperature issues. Not on recent Linux kernel versions. Linux automatically detects when the TSC is unstable (due to power management or out-of-sync cores/sockets) and automatically falls back to the more expensive HPET or ACPI methods. From the patch: Newer operating systems may check for the known TSC problems and switch to a slower, more stable clock source when they are seen. If your system supports TSC time but doesn't default to that, it may be disabled for a good reason. I ran into a case like you're showing here in my longer exploration of this at http://archives.postgresql.org/message-id/4edf1871.2010...@2ndquadrant.com I stopped just short of showing what the TSC error message looked like. I hoped that with the above and some examples showing dmesg | grep, that would be enough to lead enough people toward finding this on their own. -- 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
Re: [HACKERS] pg_test_timing tool for EXPLAIN ANALYZE overhead
On Wed, Feb 22, 2012 at 19:36, Greg Smith g...@2ndquadrant.com wrote: From the patch: Newer operating systems may check for the known TSC problems and switch to a slower, more stable clock source when they are seen. If your system supports TSC time but doesn't default to that, it may be disabled for a good reason. Sorry, I was under the impression that the stability of Windows's QueryPerformanceCounter() API is hardware-dependent, but I haven't coded under Windows for a long time -- maybe it's improved in recent versions. Regards, Marti -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
