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The OOM killer seems to be killing things despite having more than enough free RAM on my system:

Memory utilization graph (Full resolution)

IO utilization graph (Full resolution)

27 minutes and 408 processes later, the system started responding again. I rebooted it about an hour after, and soon thereafter the memory utilization returned to normal (for this machine).

Upon inspection, I've got a few interesting processes running on my box:

root      1399 60702042  0.2 482288 1868 ?     Sl   Feb21 21114574:24 /sbin/rsyslogd -i /var/run/syslogd.pid -c 4
mysql     2022 60730428  5.1 1606028 38760 ?   Sl   Feb21 21096396:49 /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock

This specific server has been running for approx. 8 hours, and these are the only two processes that have such... odd values. My suspicion is that "something else" is going on, potentially relevant to these non-sensical values. In specific, I think that the system thinks that it is out of memory, when in reality, it isn't. After all, it thinks that rsyslogd is using 55383984% CPU consistently, when the theoretical maximum is 400% on this system anyway.

This is a fully up to date CentOS 6 install (6.2) with 768MB of RAM. Any suggestions on how to figure out why this is happening would be appreciated!

edit: attaching the vm. sysctl tunables.. I have been playing with swappiness (made evident by it being 100), and I'm also running an absolutely terrible script which dumps my buffers and cache (made evident by vm.drop_caches being 3) + syncs the disk every 15 minutes. This is why after the reboot, the cached data grew to a somewhat normal size, but then quickly dropped off again. I recognize that having cache is a Very Good Thing, but until I get this figured out...

Also somewhat interesting is that while my pagefile grew during the event, it only reached ~20% of total possible utilization, which is uncharacteristic of true OOM events. On the other end of the spectrum, the disk went absolutely nuts during the same period, which is characteristic of an OOM event when the pagefile is in play.

vm.overcommit_memory = 1
vm.panic_on_oom = 0
vm.oom_kill_allocating_task = 0
vm.extfrag_threshold = 500
vm.oom_dump_tasks = 0
vm.would_have_oomkilled = 0
vm.overcommit_ratio = 50
vm.page-cluster = 3
vm.dirty_background_ratio = 10
vm.dirty_background_bytes = 0
vm.dirty_ratio = 20
vm.dirty_bytes = 0
vm.dirty_writeback_centisecs = 500
vm.dirty_expire_centisecs = 3000
vm.nr_pdflush_threads = 0
vm.swappiness = 100
vm.nr_hugepages = 0
vm.hugetlb_shm_group = 0
vm.hugepages_treat_as_movable = 0
vm.nr_overcommit_hugepages = 0
vm.lowmem_reserve_ratio = 256   256     32
vm.drop_caches = 3
vm.min_free_kbytes = 3518
vm.percpu_pagelist_fraction = 0
vm.max_map_count = 65530
vm.laptop_mode = 0
vm.block_dump = 0
vm.vfs_cache_pressure = 100
vm.legacy_va_layout = 0
vm.zone_reclaim_mode = 0
vm.min_unmapped_ratio = 1
vm.min_slab_ratio = 5
vm.stat_interval = 1
vm.mmap_min_addr = 4096
vm.numa_zonelist_order = default
vm.scan_unevictable_pages = 0
vm.memory_failure_early_kill = 0
vm.memory_failure_recovery = 1

edit: and attaching the first OOM message... upon closer inspection, it's saying that something clearly went out of its way to eat the entirety of my swap space as well.

Feb 21 17:12:49 host kernel: mysqld invoked oom-killer: gfp_mask=0x201da, order=0, oom_adj=0
Feb 21 17:12:51 host kernel: mysqld cpuset=/ mems_allowed=0
Feb 21 17:12:51 host kernel: Pid: 2777, comm: mysqld Not tainted 2.6.32-71.29.1.el6.x86_64 #1
Feb 21 17:12:51 host kernel: Call Trace:
Feb 21 17:12:51 host kernel: [<ffffffff810c2e01>] ? cpuset_print_task_mems_allowed+0x91/0xb0
Feb 21 17:12:51 host kernel: [<ffffffff8110f1bb>] oom_kill_process+0xcb/0x2e0
Feb 21 17:12:51 host kernel: [<ffffffff8110f780>] ? select_bad_process+0xd0/0x110
Feb 21 17:12:51 host kernel: [<ffffffff8110f818>] __out_of_memory+0x58/0xc0
Feb 21 17:12:51 host kernel: [<ffffffff8110fa19>] out_of_memory+0x199/0x210
Feb 21 17:12:51 host kernel: [<ffffffff8111ebe2>] __alloc_pages_nodemask+0x832/0x850
Feb 21 17:12:51 host kernel: [<ffffffff81150cba>] alloc_pages_current+0x9a/0x100
Feb 21 17:12:51 host kernel: [<ffffffff8110c617>] __page_cache_alloc+0x87/0x90
Feb 21 17:12:51 host kernel: [<ffffffff8112136b>] __do_page_cache_readahead+0xdb/0x210
Feb 21 17:12:51 host kernel: [<ffffffff811214c1>] ra_submit+0x21/0x30
Feb 21 17:12:51 host kernel: [<ffffffff8110e1c1>] filemap_fault+0x4b1/0x510
Feb 21 17:12:51 host kernel: [<ffffffff81135604>] __do_fault+0x54/0x500
Feb 21 17:12:51 host kernel: [<ffffffff81135ba7>] handle_pte_fault+0xf7/0xad0
Feb 21 17:12:51 host kernel: [<ffffffff8103cd18>] ? pvclock_clocksource_read+0x58/0xd0
Feb 21 17:12:51 host kernel: [<ffffffff8100f951>] ? xen_clocksource_read+0x21/0x30
Feb 21 17:12:51 host kernel: [<ffffffff8100fa39>] ? xen_clocksource_get_cycles+0x9/0x10
Feb 21 17:12:51 host kernel: [<ffffffff8100c949>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
Feb 21 17:12:51 host kernel: [<ffffffff8113676d>] handle_mm_fault+0x1ed/0x2b0
Feb 21 17:12:51 host kernel: [<ffffffff814ce503>] do_page_fault+0x123/0x3a0
Feb 21 17:12:51 host kernel: [<ffffffff814cbf75>] page_fault+0x25/0x30
Feb 21 17:12:51 host kernel: Mem-Info:
Feb 21 17:12:51 host kernel: Node 0 DMA per-cpu:
Feb 21 17:12:51 host kernel: CPU    0: hi:    0, btch:   1 usd:   0
Feb 21 17:12:51 host kernel: CPU    1: hi:    0, btch:   1 usd:   0
Feb 21 17:12:51 host kernel: CPU    2: hi:    0, btch:   1 usd:   0
Feb 21 17:12:51 host kernel: CPU    3: hi:    0, btch:   1 usd:   0
Feb 21 17:12:51 host kernel: Node 0 DMA32 per-cpu:
Feb 21 17:12:51 host kernel: CPU    0: hi:  186, btch:  31 usd:  47
Feb 21 17:12:51 host kernel: CPU    1: hi:  186, btch:  31 usd:   0
Feb 21 17:12:51 host kernel: CPU    2: hi:  186, btch:  31 usd:   0
Feb 21 17:12:51 host kernel: CPU    3: hi:  186, btch:  31 usd: 174
Feb 21 17:12:51 host kernel: active_anon:74201 inactive_anon:74249 isolated_anon:0
Feb 21 17:12:51 host kernel: active_file:120 inactive_file:276 isolated_file:0
Feb 21 17:12:51 host kernel: unevictable:0 dirty:0 writeback:2 unstable:0
Feb 21 17:12:51 host kernel: free:1600 slab_reclaimable:2713 slab_unreclaimable:19139
Feb 21 17:12:51 host kernel: mapped:177 shmem:84 pagetables:12939 bounce:0
Feb 21 17:12:51 host kernel: Node 0 DMA free:3024kB min:64kB low:80kB high:96kB active_anon:5384kB inactive_anon:5460kB active_file:36kB inactive_file:12kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:14368kB mlocked:0kB dirty:0kB writeback:0kB mapped:16kB shmem:0kB slab_reclaimable:16kB slab_unreclaimable:116kB kernel_stack:32kB pagetables:140kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:8 all_unreclaimable? no
Feb 21 17:12:51 host kernel: lowmem_reserve[]: 0 741 741 741
Feb 21 17:12:51 host kernel: Node 0 DMA32 free:3376kB min:3448kB low:4308kB high:5172kB active_anon:291420kB inactive_anon:291536kB active_file:444kB inactive_file:1092kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:759520kB mlocked:0kB dirty:0kB writeback:8kB mapped:692kB shmem:336kB slab_reclaimable:10836kB slab_unreclaimable:76440kB kernel_stack:2520kB pagetables:51616kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:2560 all_unreclaimable? yes
Feb 21 17:12:51 host kernel: lowmem_reserve[]: 0 0 0 0
Feb 21 17:12:51 host kernel: Node 0 DMA: 5*4kB 4*8kB 2*16kB 0*32kB 0*64kB 1*128kB 1*256kB 1*512kB 0*1024kB 1*2048kB 0*4096kB = 3028kB
Feb 21 17:12:51 host kernel: Node 0 DMA32: 191*4kB 63*8kB 9*16kB 2*32kB 0*64kB 1*128kB 1*256kB 1*512kB 1*1024kB 0*2048kB 0*4096kB = 3396kB
Feb 21 17:12:51 host kernel: 4685 total pagecache pages
Feb 21 17:12:51 host kernel: 4131 pages in swap cache
Feb 21 17:12:51 host kernel: Swap cache stats: add 166650, delete 162519, find 1524867/1527901
Feb 21 17:12:51 host kernel: Free swap  = 0kB
Feb 21 17:12:51 host kernel: Total swap = 523256kB
Feb 21 17:12:51 host kernel: 196607 pages RAM
Feb 21 17:12:51 host kernel: 6737 pages reserved
Feb 21 17:12:51 host kernel: 33612 pages shared
Feb 21 17:12:51 host kernel: 180803 pages non-shared
Feb 21 17:12:51 host kernel: Out of memory: kill process 2053 (mysqld_safe) score 891049 or a child
Feb 21 17:12:51 host kernel: Killed process 2266 (mysqld) vsz:1540232kB, anon-rss:4692kB, file-rss:128kB
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Can you provide the output of sysctl -a 2>/dev/null | grep '^vm'? –  Patrick Feb 25 '12 at 7:46
Added. Hopefully you (or someone) can find something that I've missed. –  Kyle Brantley Feb 25 '12 at 17:55
The only thing jumping out at me is the overcommit_memory setting. Setting it to 1 shouldnt cause this behavior, but I havent ever had a need to set it to 'always overcommit' before, so not much experience there. Looking at the other notes you added, you said that swap was only 20% used. However according to the OOM log dump, Free swap = 0kB. It definitely thought swap was 100% used. –  Patrick Feb 25 '12 at 18:40
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3 Answers

up vote 8 down vote accepted

I just looked at the oom log dump, and I question the accuracy of that graph. Notice the first 'Node 0 DMA32' line. It says free:3376kB, min:3448kB, and low:4308kB. Whenever the free value drops below the low value, kswapd is supposed to start swapping things until that value gets back up above the high value. Whenever free drops below min, the system basically freezes until the kernel gets it back up above the min value. That message also indicates that swap was completely used where it says Free swap = 0kB.
So basically kswapd triggered, but swap was full, so it couldnt do anything, and the pages_free value was still below the pages_min value, so the only option was to start killing things until it could get pages_free back up.
You definitely ran out of memory.

http://dd.qc.ca/people.redhat.com/kernel/min_free_kbytes.html has a really good explanation of how that works. See the 'Implementation' section at the bottom.

share|improve this answer
So then the OP really simply needs more RAM... –  ewwhite Feb 25 '12 at 19:03
more ram, or find out whats eating it. –  Patrick Feb 25 '12 at 19:10
I put the lines on that graph very precisely. It stopped logging data ~1-2m before the first process was killed, and resumed logging data ~4-5m after the last one was killed. At this point, I'm betting that some process went absolutely haywire and started thrashing my pagefile -- once it finally got everything, it killed the processes that were functionally running out of the pagefile as well, which is why when the data returned, the pagefile was elevated but not full. Suggestions on how to determine what is doing this would be welcome! –  Kyle Brantley Feb 25 '12 at 20:26
@KyleBrantley What values are you pulling to generate the graph? One of the downsides of the linux virtual memory system is that there is no concrete definition of 'free'. There are a dozen ways of defining 'free memory'. The one that matters as far as kswapd goes is the pages_free value. As for free swap, I dont know what value you could be reading which would be that far off. The only way to really see whats eating the memory is logging continual snapshots of all processes on the box and see whats using up all the memory when the oom killer starts going nuts. –  Patrick Feb 25 '12 at 20:39
Yup, I ran out of memory. I wound up grepping through the logs to find that my apache child processes were being repeatedly killed, which led me to figure out that I had functionally infinite child processes that it could start. All it took was the automated blogspam bots throwing a lot of requests/second at the host for it to fall over. Tweaking apache solved everything. –  Kyle Brantley May 13 '12 at 19:19
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There is no data available on the graph from when the OOM killer starts until it ends. I believe in the time frame where the graph is interrupted that in fact memory consumption does spike and there is no memory available anymore. Otherwise the OOM killer would not be used. If you watch the free memory graph after the OOM killer has stopped you can see it goes down from a higher value than before. At least it did its job properly, freeing up memory.

Note that your swap space is almost utilized fully until reboot. That is almost never good thing and a sure sign there is little free memory left.

The reason there is no data available for that particular time frame is because the system is too busy with other things. "Funny" values in your process list may just be a result, not a cause. It's not unheard of.

Check /var/log/kern.log and /var/log/messages, what information can you find there?

If logging also stopped then try other things, dump the process list to a file every second or so, same with the system performance information. Run it with high priority so it can still do its job (hopefully) when the load spikes. Although if you don't have a preempt kernel (sometimes indicated as a "server" kernel) you may be out of luck in that regard.

I think you will find that the process(es) that is (are) using the most CPU% around the time your problems start is (are) the cause. I have never seen rsyslogd neither mysql behaving that way. More likely culprits are java apps and gui driven apps such as a browser.

share|improve this answer
There's no data in that gap because the load on the box spikes so absurdly high that everything, including the monitoring agent, grinds to a complete halt. The agent itself was never actually killed during the period of near-death, but it could not report any data back either. My swap space was actually fine. It was using ~130/512MB total prior to the reboot. rsyslogd is configured to report logs to a network location, where everything that happened was logged -- and aside from it killing 408 processes (some of which were apache children which were restarted), nothing stood out. –  Kyle Brantley Feb 24 '12 at 6:21
I may wind up dumping the complete process list to a file (or the network) every few seconds if I can't actually figure out what is going on here though, so thanks for the good idea. –  Kyle Brantley Feb 24 '12 at 6:21
Yeah you should do that, make sure you also record CPU usage of each process, take look at "top -b" (batch mode). The process that spikes could be the culprit. –  aseq Feb 24 '12 at 19:16
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Get rid of the drop_caches script. In addition, you should post the relevant portions of your dmesg and /var/log/messages output showing the OOM messages.

To stop this behavior, though, I'd recommend trying this sysctl tunable. This is a RHEL/CentOS 6 system and is clearly running on constrained resources. Is it a virtual machine?

Try modifying /proc/sys/vm/nr_hugepages and see if the issues persist. This could be a memory fragmentation issue, but see if this setting makes a difference. To make the change permanent, add vm.nr_hugepages = value to your /etc/sysctl.conf and run sysctl -p to reread the config file...

Also see: Interpreting cryptic kernel "page allocation failure" messages

share|improve this answer
Added the first oom-killer message. While MySQL is the most RAM intensive thing I'm running, I've tuned it down as not to overwhelm the box either, so aside from the crazy values I'm seeing in top, I'm not really suspecting it (5.1% memory usage is fine, all things considered). It is a VPS, with 768MB of RAM. I'll read up on the nr_hugepages and give it a shot, thanks! –  Kyle Brantley Feb 25 '12 at 18:09
@ewwhite allocating that many hugepages would kill the box. The box only has 768mb ram, and with the default hugepagesz of 2mb, that would allocate 2gb of hugepages. The box wouldnt be able to handle that and would immediately die. –  Patrick Feb 25 '12 at 18:48
Updated. The value should be increased from zero, though. –  ewwhite Feb 25 '12 at 18:52
Its still more complicated than that. You have to set the huge page permissions, and mysql must be configured to use huge pages, otherwise youre just allocating them for no reason. –  Patrick Feb 25 '12 at 19:07
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