My Ubuntu 12 server is mysteriously losing/wasting memory. It has 64GB of ram. About 46GB are shown as used even when I shutdown all my applications. This memory is not reported as used for buffers or caching.
The result of top (while my apps are running; the apps use about 9G):
top - 21:22:48 up 46 days, 10:12, 1 user, load average: 0.01, 0.09, 0.12
Tasks: 635 total, 1 running, 633 sleeping, 1 stopped, 0 zombie
Cpu(s): 0.2%us, 0.2%sy, 0.0%ni, 99.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 65960100k total, 55038076k used, 10922024k free, 271700k buffers
Swap: 0k total, 0k used, 0k free, 4860768k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5303 1002 20 0 26.2g 1.2g 12m S 0 1.8 2:08.21 java
5263 1003 20 0 9.8g 995m 4544 S 0 1.5 0:19.82 mysqld
7021 www-data 20 0 3780m 18m 2460 S 0 0.0 8:37.50 apache2
7022 www-data 20 0 3780m 18m 2540 S 0 0.0 8:38.28 apache2
.... (smaller processes)
Note that top reports 4.8G for cached, not 48G, and it's 55G that are used. The result of free -m:
total used free shared buffers cached
Mem: 64414 53747 10666 0 265 4746
-/+ buffers/cache: 48735 15678
Swap: 0 0 0
What is using my memory? I've tried every diagnostic that I could come across. Forums are swamped with people asking the same question because Linux is using their ram for buffers/cache. This doesn't seem to be what is going on here.
It might be relevant that the system is a host for lxc containers. The top and free results reported above are from the host, but similar memory usage is reported within the containers. Stopping all containers does not free up the memory. Some 46G remain in use. However, if I restart the host the memory is free. It doesn't reach the 46G before a while. (I don't know if it takes days or weeks. It takes more than a few hours.)
It might also be relevant that the system is using zfs. Zfs is reputed memory-hungry, but not that much. This system has two zfs filesystems on two raidz pools, one of 1.5T and one of 200G. I have another server that exhibits exactly the same problem (46G used by nothing) and is configured pretty much identically, but with a 3T array instead of 1.5T. I have lots of snapshots (100 or so) for each zfs filesystem. I normally have one snapshot of each filesystem mounted at any time. Unmounting those does not give me back my memory.
I can see that the VIRT numbers in the screenshot above coincide roughly with the memory used, but the memory remains used even after I shutdown these apps--even after I shutdown the container that's running them.
EDIT: I tried adding some swap, and something interesting happened. I added 30G of swap. Moments later, the amount of memory marked as cached in top had increased from 5G to 25G. Free -m indicated about 20G more usable memory. I added another 10G of swap, and cached memory raised to 33G. If I add another 10G of swap, I get 6G more recognized as cached. All this time, only a few kilobytes of swap are reported used. It's as if the kernel needed to have matching swap for every bit that it recognizes or reports as cached. Here is the output of top with 40G of swap:
top - 23:06:45 up 46 days, 11:56, 2 users, load average: 0.01, 0.12, 0.13
Tasks: 586 total, 1 running, 585 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 65960100k total, 64356228k used, 1603872k free, 197800k buffers
Swap: 39062488k total, 3128k used, 39059360k free, 33101572k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6440 1002 20 0 26.3g 1.5g 11m S 0 2.4 2:02.87 java
6538 1003 20 0 9.8g 994m 4564 S 0 1.5 0:17.70 mysqld
4707 dbourget 20 0 27472 8728 1692 S 0 0.0 0:00.38 bash
Any suggestions highly appreciated.
EDIT 2: Here are the arc* values from /proc/spl/kstat/zfs/arcstats
arc_no_grow 4 0
arc_tempreserve 4 0
arc_loaned_bytes 4 0
arc_prune 4 0
arc_meta_used 4 1531800648
arc_meta_limit 4 8654946304
arc_meta_max 4 8661962768
There is no L2ARC activated for ZFS
This memory is very likely used by the ZFS ARC cache and other ZFS related data stored in the kernel memory. The ARC cache is somewhat similar to the buffer cache so there is generally nothing to worry about it as this memory is released by ZFS should there is demand to it.
However, there is a subtle difference between buffer cache memory and ARC cache one. The first one is immediately available to allocation while the ARC cache one is not. ZFS monitors the free RAM available and when too low, it releases RAM to other consumers.
This works fine with most applications but a minority of them are either confused when a low amount of available RAM is reported, or allocate too much/too fast memory for the release process to keep up the pace properly.
That's the reason why ZFS allows to reduce the maximum size the ARC size is allowed to use.
This setting is done in the /etc/modprobe.d/zfs.conf file.
For example, should you want the ARC never to exceed 32 GB, add this line:
options zfs zfs_arc_max=34359738368
To get the current ARC size and various other ARC statistics, run this command:
cat /proc/spl/kstat/zfs/arcstats
The size metric will show the current size of the ARC. Beware that other ZFS related memory areas might also take a share of RAM and won't be necessarily quickly released even when no more used. Finally, ZFS on linux is certainly less mature than the Solaris native implementation so you might be hit by a bug like this one.
Note too that due to the share storage pool design, unmounting a ZFS file system won't free any resource. You would need to export a pool for memory to be eventually released.
Related
we run a debian server with 64Gb of RAM to run large python simulations.
The problem we face is that a large amount of this memory is getting used and we don't know why or how to correct that.
It appears it is not a cache/buffer thing:
free -m
total used free shared buffers cached
Mem: 64454 56243 8211 20 6 113
-/+ buffers/cache: 56122 8332
Swap: 21051 5834 15217
When running smem, it shows us that after a few days, up to 37 Gb are allocated for the kernel dynamic memory.
Area Used Cache Noncache
firmware/hardware 0 0 0
kernel image 0 0 0
kernel dynamic memory 36.8G 431.0M 36.4G
userspace memory 4.5G 149.7M 4.4G
free memory 21.6G 21.6G 0
----------------------------------------------------------
62.9G 22.2G 40.8G
We rebooted the server yesterday, and while a the start it shows a kernel dynamic memory of 1.5 Gb, it slowly increases.
24 hours later, it has already reached 17Gb
Area Used Cache Noncache
firmware/hardware 0 0 0
kernel image 0 0 0
kernel dynamic memory 17.1G 269.3M 16.8G
userspace memory 36.4G 73.0M 36.3G
free memory 9.4G 9.4G 0
----------------------------------------------------------
62.9G 9.8G 53.2G
Any idea how to investigate further and if this is really a memory leak, what should we do? (kernel is 3.16)
Thanks in advance
Trying to understand what's going on with my server.
It's a 2 cpu server, so:
$> grep 'model name' /proc/cpuinfo | wc -l
2
While on load avergae, queue is showing ~8 :
$> uptime
16:31:30 up 123 days, 9:04, 1 user, load average: 8.37, 8.48, 8.55
So You can assume, load is really high and things are pailing up, there is some load on the system and it's not just a spike.
However, Looking at top cpu consumers:
> ps -eo pcpu,pid,user,args | sort -k 1 -r | head -6
%CPU PID USER COMMAND
8.3 27187 **** server_process_c
1.0 22248 **** server_process_b
0.5 22282 **** server_process_a
0.0 31167 root head -6
0.0 31166 root sort -k 1 -r
0.0 31165 root ps -eo pcpu,pid,user,args
Results of free command:
total used free shared buffers cached
Mem: 7986 7934 52 0 9 2446
-/+ buffers/cache: 5478 2508
Swap: 17407 60 17347
This is the result on an ongoing basis, e.g. not even
a single CPU is being used, top consumer, is always ~8.5%.
My Question: What are my ways to track down the root of the high load?
Based on your free output, there are times when system memory is exhausted so swap buffer is used (see column used = 60). Total memory used used - (buffers + cached) which result almost zero. It means there are time when all physical RAM is consumed.
For server, try to avoid page fault which may cause swapping data from system memory to swap buffer (or vice versa) as much as possible because accessing hard drive is very slow than system RAM.
In your top output, try to investigate wa column. Higher percentage value means CPU spend more times waiting for data IO from disk rather than doing meaningful computation.
Cpu(s): 87.3%us, 1.2%sy, 0.0%ni, 27.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Try to reduce daemon or service that you do not need to reduce memory footprint and consider to add more RAM to the system.
For 2 CPU(s) server, ideal load is less than 2.0 (each CPU load is less than 1.0). Load of 8.0 means each CPU load is roughly 4.0 which is not very good.
Have you tried the htop command? It shows more information in a helpful way sometimes.
I was running an application which was to load about 60 mil items in memcache. I had two servers added in a bucket. After about 65% of the data was loaded, I saw 1.3 mil items evicted in both servers. And these were statistics at that point.
On server 1
STAT bytes_written 619117542
STAT limit_maxbytes 3145728000
On server 2
STAT bytes_written 619118863
STAT limit_maxbytes 3145728000
Here's the output of free -m at that point of time.
On server 1
total used free shared buffers cached
Mem: 7987 5965 2021 0 310 441
-/+ buffers/cache: 5213 2774
Swap: 4095 0 4095
On sever 2
total used free shared buffers cached
Mem: 11980 11873 106 0 207 5860
-/+ buffers/cache: 5805 6174
Swap: 5119 0 5119
As we can see, on both servers, limit_maxbytes was not reached. Only about 600MB was used at both the places. However on server 2, free memory dipped to as low as 100 mb. Now I know that cached is 5.8 GB and that linux could free that memory for running processes. But it looks like that didn't happen and seeing memory reaching critical level, memcached started evicting items.
Or is there any other reason? When exactly does linux free up cache memory? Is 100 mb of free ram is still not critical enough for linux to free up cache? Please help me understanding why such an even occured.
The 'slabs' refer to how Memcached allocates memory. Rather than a complex exact-match,it puts your data into a close-enough (slightly larger) piece of memory within the server. This means that it will frequently 'waste' memory that isn't storing your data.
You can tweak how big each potential slot is though when you start the memcached server with the factor (-f) and the initial chunk-size (-s) options. How you set those, depends on the mix of sizes you are storing in cache.
I am using "free -m -t " command to monitor my linux system and get
total used free shared buffers cached
Mem: 64334 64120 213 0 701 33216
-/+ buffers/cache: 30202 34131
Swap: 996 0 996
Total: 65330 64120 1209
it means 30GB of physical memory is used by user processes.
but when using top command and sort by memory usage, only 3~4GB of memory is used by all the application processes.
Why does this inconsistency happen?
As I understand it, the amount of memory that top shows as used includes cold memory from older processes that are not running anymore. This is due to the fact that in case of a restart of said process, the required data may still be in memory, enabling the system to start the process faster and more efficiently instead or always reloading the data from disk.
or, in short, linux generally frees cold data in memory as late as possible.
Hope that clears it up :)
According to the result of "top" command, the memory usage experienced increasing a lot. However, the according to the memory usage of each processes, there are not so much memory used.
top - 19:57:01 up 1 day, 19:24, 6 users, load average: 0.17, 0.22, 0.57
Tasks: 500 total, 1 running, 499 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.1%us, 0.1%sy, 0.0%ni, 99.8%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 98858004k total, 90524192k used, 8333812k free, 1039700k buffers
Swap: 16777208k total, 0k used, 16777208k free, 72896460k cached
It seems that the Linux will mark buffers/cache memory usage as used, but they are not used by applications. In this case, I still have a log memory available.
However, the memory and swap was then exhausted, which causes the failure of the server. Is it possible that this is a memory leak of some applications?
If encountering a memory leak, will the memory usage be calculated into the process?
free -m
total used free shared buffers cached
Mem: 96541 88445 8095 0 1016 71223
-/+ buffers/cache: 16205 80335
Swap: 16383 0 16383
If encountering a memory leak, will the memory usage be calculated into the process?
Yes, a memory leak is just data who isnt needed anymore but hasnt gotten freed free(trash);
However, the memory and swap was then exhausted, which causes the failure of the server. Is it possible that this is a memory leak of some applications?
Yes, monitor wich program uses somuch RAM and debug it with http://valgrind.org/
TL;DR
As long as the leak isn't in the kernel, any leaked memory should be accounted against the application's process.
Linux and Free Memory
Linux is very aggressive about caching, and tends to allocate a lot of available memory for caching filesystem blocks and storing buffers. It's very dynamic, so this space is still available to applications if they need it, but in the meantime Linux tries to put it to use.
In other words, don't assume that a high percentage of memory used is a symptom of memory leaks. It's business as usual for the kernel.
See Also
http://linux.about.com/od/lsa_guide/a/gdelsa44.htm
Make sure /proc/sys/vm/overcommit_memory is 0. If it isn't it's much harder to spot runaway memory eating processes.