>> "Swapping excessively" is inherently a use-case-specific problem, but it >> comes >> down to two questions: >> >> * Do you notice your system spending time in iowait swapping data in while >> you're waiting on it? >> * Do you notice your system spending time in iowait swapping data out while >> you're waiting on it? (I.e. as it tries to make room for new memory >> allocations)
I just ran sar from the sysstat package and this looks like a lot of iowait to me: 00:00:02 CPU %user %nice %system %iowait %steal %idle 00:10:01 all 48.11 0.86 0.83 1.38 0.00 48.82 00:20:01 all 43.98 0.85 0.64 0.54 0.00 53.99 00:30:01 all 48.17 0.90 1.04 0.82 0.00 49.07 00:40:01 all 48.69 0.85 1.06 0.48 0.00 48.92 00:50:01 all 49.74 0.87 0.58 0.49 0.00 48.33 01:00:01 all 46.21 0.85 0.48 0.41 0.00 52.05 01:10:01 all 48.10 0.86 0.79 0.61 0.00 49.64 01:20:01 all 54.00 0.86 0.60 0.65 0.00 43.89 01:30:01 all 45.81 0.85 0.49 0.49 0.00 52.36 01:40:01 all 52.04 0.86 0.56 0.56 0.00 45.99 01:50:01 all 48.49 0.85 0.52 0.47 0.00 49.66 02:00:01 all 43.18 0.85 0.48 0.50 0.00 54.99 02:10:01 all 45.48 1.12 1.74 20.65 0.00 31.01 02:20:02 all 46.20 10.22 1.97 9.70 0.00 31.90 02:30:01 all 64.93 0.88 1.98 12.54 0.00 19.67 02:40:01 all 46.24 0.86 0.93 5.08 0.00 46.90 02:50:01 all 43.49 0.85 0.45 0.60 0.00 54.60 03:00:01 all 43.28 0.85 0.45 0.45 0.00 54.97 03:10:01 all 39.58 0.85 0.81 5.22 0.00 53.54 03:20:01 all 42.04 0.91 0.72 3.97 0.00 52.35 03:30:01 all 46.60 0.85 0.74 0.49 0.00 51.31 03:40:01 all 47.30 0.85 0.82 0.82 0.00 50.22 03:50:01 all 49.43 0.85 0.84 0.59 0.00 48.29 04:00:01 all 45.50 0.85 1.02 0.71 0.00 51.91 04:10:01 all 44.35 0.86 1.13 2.32 0.00 51.35 04:20:01 all 44.29 0.85 1.17 4.91 0.00 48.77 04:30:01 all 42.69 0.85 0.47 1.41 0.00 54.59 04:40:01 all 48.22 0.85 1.00 7.23 0.00 42.70 04:50:01 all 44.70 0.86 0.49 1.49 0.00 52.45 Average: all 46.92 1.19 0.86 2.95 0.00 48.08 > If I do find a correlation between iowait and web server response > times, should I just decrease memory usage until the problem goes > away? > > What I do notice is that my web server's response time increases along > with the swapping peaks in the graph I posted before. > > >> There are ways other than swap to find yourself in iowait, though. I wonder >> what might a good metric of combining iowait numbers with swap event counts. >> Swap events without iowait are likely imperceptible. I do see a clear correlation between iowait above and swap in on the munin graph. Is that enough to conclude that swap activity is slowing down the system and I need to reduce memory usage or perhaps tune swappiness? - Grant
