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Message-ID: <20090604062122.GA8126@untroubled.org>
Date: Thu, 4 Jun 2009 00:21:22 -0600
From: Bruce Guenter <bruce@...roubled.org>
To: linux-kernel@...r.kernel.org
Subject: Probablem with dropping caches
Hello.
I am having a problem with a system that appears to be spontaneously
dropping large parts of its caches. The work load on this system is
primarily I/O bound (it's a mailbox server), and as such the loss of
cache memory is causing severe performance degradation.
For example, here is some output from vmstat 1:
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 1 0 87164 683212 1069748 0 0 544 164 856 730 3 2 91 4
0 1 0 81124 689100 1069508 0 0 5880 104 1070 834 2 1 74 23
0 1 0 89956 691588 1057408 0 0 5288 0 1163 915 0 2 72 25
0 1 0 138020 690652 1012444 0 0 5724 0 1136 831 2 0 75 23
0 0 0 243384 690460 906500 0 0 4716 0 1282 844 0 2 61 36
0 1 0 294704 690152 854232 0 0 1108 428 1123 1093 2 2 81 15
0 0 0 285984 690380 854504 0 0 252 0 721 671 3 1 92 3
0 1 0 426844 690780 722408 0 0 3096 1748 1197 846 1 2 84 13
0 1 0 579684 691232 568344 0 0 4228 156 1300 1083 2 2 69 27
1 1 0 676312 691832 467244 0 0 5256 0 1072 741 0 2 75 23
As far as I can tell from df and similar reporting, there are not
hundreds of MB of files being deleted, which would have similar
behavior. It is not swapping, nor is memory actually leaking (since
free memory + cache is nearly constant). All of the active programs run
with small memory ulimits and as such are not consuming and then
releasing hundreds of MB of memory.
There are also intervals where the system is reading several MB per
second but the caches do not grow significantly:
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 35 0 960396 749544 62416 0 0 7396 0 1424 1154 2 2 0 96
1 34 0 963868 750536 62252 0 0 8596 208 1695 1463 4 3 0 93
0 38 0 967452 752800 62980 0 0 7176 20 1378 972 4 1 0 95
0 38 0 968100 751308 61400 0 0 7260 180 1423 1109 3 2 0 95
2 42 0 966252 751540 61872 0 0 8196 0 1404 1328 1 1 0 97
0 43 0 955440 751956 60520 0 0 8692 0 1846 1925 5 3 0 92
2 49 0 943644 752836 61412 0 0 9324 200 1783 1582 5 3 0 92
1 39 0 959368 751892 62104 0 0 7836 64 1874 1855 9 5 0 86
This system has 2GB RAM and 4 72GB drives in a 3Ware RAID10 array. The
active filesystem is ext4 with the following mount options:
noatime,nodiratime,data=journal
The data=journal option comes from benchmarking I did a while back that
indicated it was best for sync+unlink heavy work loads such as this one
has. I have remounted with data=ordered but that did not solve the
problem.
The kernel (as of now) is 2.6.29.4 compiled with gcc 3.4.6 on Gentoo.
I also have another system, which is similarly configured but is using
the ext3 filesystem. It does not exhibit this behavior which leads me
to suspect some difference between ext3 and ext4 is causing the problem.
I however have no other evidence to point a finger at ext4, and am at a
loss as to what else to investigate.
Has anybody else seen this behavior before? What other details can I
investigate to figure out what is causing this problem? What other information
would be useful to diagnose this?
Thank you.
--
Bruce Guenter <bruce@...roubled.org> http://untroubled.org/
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