[erlang-questions] os_mon memsup reporting memory consumption

[Gunilla Arendt]

Hi

I see the point, but it would be a non-backward compatible change
and as you know we're very hesitant about those.

An alternative solution could be to add a new tag to the result of
memsup:get_system_memory_data() for Linux systems, although that would
not affect the setting/clearing of alarms.

If anybody else has thoughts about this, please feel free to share.

Regards,
Gunilla, Erlang/OTP team



Serge Aleynikov wrote:
> We had a few high memory watermark alarms in a production system running 
> Linux reported by OS_MON whereas it seemed like the memory utilization 
> on the server wasn't bad.  I looked at the implementation of the 
> memsup.erl module in OS_MON, and I believe the calculation of allocated 
> memory has a flow.
> 
> Memsup's code (sited at the end) reveals that allocated memory doesn't 
> take in consideration cached memory.  Since cached memory can be quickly 
> freed by the Linux memory manager if additional memory is needed by 
> applications, it looks to me that it should be:
> 
> Allocated = MemTotal - (MemFree + Cached)
> 
> or even:
> 
> Allocated = MemTotal - (MemFree + Buffers + Cached)
> 
> This would be consistent with what "free" is showing on the "-/+ 
> buffers/cache" line:
> 
> drp@REDACTED: ~$ uname -a
> Linux drpdb02.corp.idt.net 2.6.9-5.ELsmp #1 SMP Wed Jan 5 19:30:39 EST 
> 2005 i686 i686 i386 GNU/Linux
> 
> drp@REDACTED: ~$ free
>              total      used       free     shared    buffers     cached
> Mem:       4150324   3416788     733536          0     282412    2621788
> -/+ buffers/cache:    512588    3637736
> Swap:      2048248         0    2048248
> 
> drp@REDACTED: ~$ head -4 /proc/meminfo
> MemTotal:      4150324 kB
> MemFree:        733536 kB
> Buffers:        282412 kB
> Cached:        2621788 kB
> 
> 
> 
> %-------------------------
> %% @spec get_memory_usage_linux() -> {Allocated, Total}
> %-------------------------
> get_memory_usage_linux() ->
>     Res = os:cmd("cat /proc/meminfo"),
>     case get_memory_usage_linux(Res, undef, undef) of
>     {MemTotal, MemFree} ->
>         {MemTotal-MemFree, MemTotal};
>     error ->
>         timer:sleep(1000),
>         get_memory_usage_linux()
>     end.
> 
> get_memory_usage_linux(_Str, Tot, Free) when is_integer(Tot),
>                          is_integer(Free) ->
>     {Tot, Free};
> get_memory_usage_linux("MemTotal:"++T, _Tot0, Free0) ->
>     {ok, [N], Rest} = io_lib:fread("~d", T),
>     Tot = N*1024,
>     get_memory_usage_linux(skip_to_eol(Rest), Tot, Free0);
> get_memory_usage_linux("MemFree:"++T, Tot0, _Free0) ->
>     {ok, [N], Rest} = io_lib:fread("~d", T),
>     Free = N*1024,
>     get_memory_usage_linux(skip_to_eol(Rest), Tot0, Free);
> get_memory_usage_linux("", _Tot0, _Free0) ->
>     error;
> get_memory_usage_linux(Str, Tot0, Free0) ->
>     get_memory_usage_linux(skip_to_eol(Str), Tot0, Free0).
> %-------------------------
> 
> Regards,
> 
> Serge
>