Thread: Context switching and Xeon processors
We're running a dual Xeon machine with hyperthreading enabled and PostgreSQL 8.0.3. Below is the type of CPUs: processor : 3 vendor_id : GenuineIntel cpu family : 15 model : 4 model name : Intel(R) Xeon(TM) CPU 3.20GHz stepping : 1 cpu MHz : 3200.274 cache size : 1024 KB ... We've been tuning the kernel (2.4 SMP flavor) and have improved performance quite a bit. I'm now wondering if turning off HT will improve performance even more. Based on the vmstat output below, is the context switching typical or too high? And what is the latest on the state of PostgreSQL running on Xeon processors with HT turned on? I searched the archives, but couldn't discern anything definitive. r b swpd free buff cache si so bi bo in cs us sy wa id 1 0 135944 64612 17136 3756816 0 0 0 210 154 178 2 0 4 94 1 0 135940 46600 17204 3754496 0 0 1 1231 442 3658 7 3 10 80 1 3 135940 51228 17240 3754680 0 0 0 1268 255 2659 4 1 14 81 1 0 135940 58512 17300 3754684 0 0 0 1818 335 1526 2 1 32 65 1 1 135940 18104 17328 3806516 0 0 17670 476 1314 1962 2 2 41 56 0 1 135940 17776 17232 3811620 0 0 23193 394 1600 2097 2 2 53 44 0 1 135940 17944 17188 3809636 0 0 25459 349 1547 2013 2 2 50 46 0 3 135940 18816 15184 3798312 0 0 24284 1328 1529 4730 6 5 53 36 0 6 135940 23536 6060 3817088 0 0 27376 1332 1350 2628 2 3 56 39 0 5 135940 18008 6036 3827132 0 0 18806 1539 1410 1416 1 2 61 36 0 5 135940 18492 5708 3826660 0 0 3540 10354 736 955 2 2 76 20 0 3 135940 18940 5788 3829864 0 0 2308 7506 707 519 2 1 81 15 1 4 135940 18980 5820 3828836 0 0 138 3503 556 261 1 0 74 24 0 10 135940 39332 5896 3777724 0 0 579 2805 621 4104 7 4 54 35 0 4 135936 37816 5952 3791404 0 0 260 1887 384 1574 2 1 40 57 0 5 135936 29552 5996 3802260 0 0 290 1642 434 1944 3 1 38 58 -- Brandon
"Brandon Metcalf" <bmetcalf@nortel.com> writes:
> We've been tuning the kernel (2.4 SMP flavor) and have improved
> performance quite a bit. I'm now wondering if turning off HT will
> improve performance even more. Based on the vmstat output below, is
> the context switching typical or too high?
Given that your CPU usage is hovering around 2%, it's highly unlikely
that you'll be able to measure any change at all by fiddling with HT.
What you need to be working on is disk I/O --- the "80% wait" number
is what should be getting your attention, not the CS number.
(FWIW, on the sort of hardware you're talking about, I wouldn't worry
about CS rates lower than maybe 10000/sec --- the hardware can sustain
well over 10x that.)
regards, tom lane
t == tgl@sss.pgh.pa.us writes: t> "Brandon Metcalf" <bmetcalf@nortel.com> writes: t> > We've been tuning the kernel (2.4 SMP flavor) and have improved t> > performance quite a bit. I'm now wondering if turning off HT will t> > improve performance even more. Based on the vmstat output below, is t> > the context switching typical or too high? t> Given that your CPU usage is hovering around 2%, it's highly unlikely t> that you'll be able to measure any change at all by fiddling with HT. t> What you need to be working on is disk I/O --- the "80% wait" number t> is what should be getting your attention, not the CS number. t> (FWIW, on the sort of hardware you're talking about, I wouldn't worry t> about CS rates lower than maybe 10000/sec --- the hardware can sustain t> well over 10x that.) Yes, I agree the disk I/O is an issue and that's what we've been addressing with the tuning we've been doing and have been able to improve. I think that we really need to go to a RAID 10 array to address the I/O issue, but thought I would investigate the context switching issue. Thanks for the information. -- Brandon
On Tue, Dec 06, 2005 at 03:01:02PM -0600, Brandon Metcalf wrote: > We're running a dual Xeon machine with hyperthreading enabled and > PostgreSQL 8.0.3. The two single most important things that will help you with high rates of context switching: - Turn off hyperthreading. - Upgrade to 8.1. /* Steinar */ -- Homepage: http://www.sesse.net/