Memory usage over quota on Couchbase-server 3.0.1


#1

Hi,

I’m running a Couchbase cluster, in 3.0.1 community edition, where I see the memory usage growing every day, over the quota defined per server. I tried to find out which part is eating the memory, but any Couchbase graph reports a so high usage. Here is an example of what I’m talking about. In my cluster, the per RAM server quota is set to 71Go.

On a particular node, I see this reports, from Couchbase graphs:

  • Meta data usage: 24 Go
  • User RAM usage: 45 Go
  • Global RAM usage: 51 Go

And from the system, a pmap -d on memcached process report 77 Go used by this processus:

> pmap -d 1412
> ...
> 00000000016de000 77381432 rw--- 0000000000000000 000:00000   [ anon ]
> ...
> mapped: 77799464K    writeable/private: 77710396K    shared: 0K

There is 6 Go difference between the quota and the memory really used, not reported by Couchbase, but only by the system, and it’s still growing. The system is running a Linux 3.16.0-4 under Debian. I disabled transparent huge pages, and set the swapiness to 0.

Have you any idea of the origin of the problem ?
Thanks in advance for the help.


TCMalloc don't release memory
#2

Are you looking at Virtual (VSZ) or Resident (RSS)? Much like many applications the virtual memory footprint of the memcached process may well be higher than the quota, but the RSS should be within the Server Quota.

Examine top, or run something like the following to see the RSS and VSZ of the memcached process:

ps -o rss,vsz -p $(pgrep memcached)

For example on a test machine which is configured for 300MB quota I see (values in KB):

$ ps -o rss,vsz -p $(pgrep memcached)
  RSS    VSZ
178356 506520

i.e. while the virtual memory size is 494MB, the actual resident is 174MB.


#3

Thanks for your reply. I see what you mean, unfortunately, RSS and VSZ are almost the same:

ps -o rss,vsz -p $(pgrep memcached)
RSS    VSZ
77091732 77799472

#4

Can you take a look at the memory statistics?

cbstats HOSTNAME:11210 memory

and

cbstats HOSTNAME:11210 allocator

replacing HOSTNAME with your node name (or just localhost if running cbstats locally)?


#5

Here is the result of the two commands:

For allocation:

MALLOC:    57461808912 (54799.9 MiB) Bytes in use by application
MALLOC: +  11297824768 (10774.4 MiB) Bytes in page heap freelist
MALLOC: +   9862720336 ( 9405.8 MiB) Bytes in central cache freelist
MALLOC: +       524256 (    0.5 MiB) Bytes in transfer cache freelist
MALLOC: +     27055552 (   25.8 MiB) Bytes in thread cache freelists
MALLOC: +    311734432 (  297.3 MiB) Bytes in malloc metadata
MALLOC:   ------------
MALLOC: =  78961668256 (75303.7 MiB) Actual memory used (physical + swap)
MALLOC: +    269885440 (  257.4 MiB) Bytes released to OS (aka unmapped)
MALLOC:   ------------
MALLOC: =  79231553696 (75561.1 MiB) Virtual address space used
MALLOC:
MALLOC:        4123541              Spans in use
MALLOC:             38              Thread heaps in use
MALLOC:           8192              Tcmalloc page size
------------------------------------------------
Call ReleaseFreeMemory() to release freelist memory to the OS (via madvise()).
Bytes released to the OS take up virtual address space but no physical memory.
------------------------------------------------
Total size of freelists for per-thread caches,
transfer cache, and central cache, by size class
------------------------------------------------
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class   3 [       32 bytes ] :   855193 objs;  26.1 MiB;  28.3 cum MiB
class   4 [       48 bytes ] :   184196 objs;   8.4 MiB;  36.8 cum MiB
class   5 [       64 bytes ] :    11118 objs;   0.7 MiB;  37.4 cum MiB
class   6 [       80 bytes ] :    13394 objs;   1.0 MiB;  38.5 cum MiB
class   7 [       96 bytes ] :  2063127 objs; 188.9 MiB; 227.3 cum MiB
class   8 [      112 bytes ] :   577069 objs;  61.6 MiB; 289.0 cum MiB
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class  72 [   139264 bytes ] :       10 objs;   1.3 MiB; 8559.4 cum MiB
class  75 [   163840 bytes ] :        2 objs;   0.3 MiB; 8559.7 cum MiB
------------------------------------------------
PageHeap: 119 sizes; 10774.4 MiB free;  257.4 MiB unmapped
------------------------------------------------
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For memory:

 bytes:                               57365397968
 ep_blob_num:                         30098210
 ep_blob_overhead:                    1514622591
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 ep_max_size:                         73400320000
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 ep_mem_tracker_enabled:              true
 ep_oom_errors:                       0
 ep_overhead:                         2758090214
 ep_storedval_num:                    362772373
 ep_storedval_overhead:               1514622591
 ep_storedval_size:                   29021789840
 ep_tmp_oom_errors:                   0
 ep_value_size:                       25498739888
 mem_used:                            57365397968
 tcmalloc_current_thread_cache_bytes: 27647072
 tcmalloc_max_thread_cache_bytes:     33554432
 total_allocated_bytes:               57452351696
 total_fragmentation_bytes:           9894530864
 total_free_mapped_bytes:             11303051264
 total_free_unmapped_bytes:           269885440
 total_heap_bytes:                    78919819264

With the exact value from the RSS (77091732/1024/1024), I got 73.5 Go, which is a smaller difference, but still above the limit. Maybe there is a small overhead which explain that. The growth is slow, maybe it will finally stops, but I have to wait to know that.


#6

See this section from the TCMalloc allocator stats:

TCMalloc (the memory allocator used by Couchbase) doesn’t immediately return memory to the OS, in case it needs the memory again in the near future (and it would have been a waste of time to return it to have to then re-allocate it).

As you can see from that extract; there’s 19.7GB in TCMalloc’s freelists; should account for the difference you are seeing. That’s quite a large amount, TCMalloc doesn’t normally keep back such a quantity of memory. Has the node been up a long time?

In terms of resolving this you can always swap-rebalance out the node with a new one (and then re-insert the original node after restarting couchbase-server if you wish) - this will clear out the TCMalloc heap.

Another option is an (undocumented, unsupported) method to request TCMalloc to release it’s freelist memory which you could look into. It’s unsupported as it can have an impact on the performance of any future writes to the database as the memory allocator may not have any already free memory available). Additionally it’s not tested by Couchbase QA afiak.

If you decide to give it a try, you’ll need to run the mcctl command on the specific node(s):

/opt/couchbase/bin/mcctl -h localhost:11210 set release_free_memory

Note that this only requests TCMalloc to release memory, it doesn’t always necessarily release all free memory…


TCMalloc don't release memory
#7

OK, thanks the detailled explaination, it explains where the missing RAM space is used. The server is not running for a long time, I restarted the whole cluster on tuesday with consecutive rebalance, to release the memory. I think TCMalloc doesn’t release the memory because of the heavy traffic received by the cluster. I create tens of millions new documents per day, and compaction is running most of the time. Currently only write operations are performed, no read.

Thanks for the tips to force TCMalloc to release memory, it can help, I will test it if the memory usage don’t stop growing.


#8

Memory usage is stable for a while. Memory overhead usage is explained, thanks again for your help.


#9

We seems to have similar VSZ crazyness:

ps -o pid,rss,vsz,args:200 -p $(pgrep beam.smp) PID RSS VSZ COMMAND 17891 2756708 666722476 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -A 16 -sbt u -P 327680 -K true -swt low -MMmcs 30 -e10240 18042 26888 889340 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -P 327680 -K true -- -root /opt/couchbase/lib/erlang -progname 37400 27348 1348780 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -A 16 -- -root /opt/couchbase/lib/erlang -progname erl -- -ho

The machine have 32Gb:

free -m total used free shared buffers cached Mem: 32170 31668 501 30 325 15336 -/+ buffers/cache: 16007 16162 Swap: 16143 0 16143

So the first process seems to use 600Gb of virtual memory…this is something really really strange…

This is a node from a 4 node cluster, other nodes have similar behaviour, one of the node I restarted this morning have:

ps -o pid,rss,vsz,args:400 -p $(pgrep beam.smp) PID RSS VSZ COMMAND 40502 25912 1282068 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -A 16 -- -root /opt/couchbase/lib/erlang -progname erl -- -ho 40536 2148212 481494160 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -A 16 -sbt u -P 327680 -K true -swt low -MMmcs 30 -e10240 40632 27004 889596 /opt/couchbase/lib/erlang/erts-5.10.4/bin/beam.smp -P 327680 -K true -- -root /opt/couchbase/lib/erlang -progname