[erlang-questions] How to tune erlang memory allocator to use 25 GB of ram and stop munmaping it?

Sverker Eriksson sverker.eriksson@REDACTED
Wed Aug 14 16:35:57 CEST 2013

Max Lapshin wrote:
> Right now I fight with problem of streaming 10 Gbit/s of video and more
> from single server.
> I have met different problems (mostly about locking into single process
> like with file:read_file),
> but right now I have one question, it is about memory allocation.
> I've started strace against one of threads running scheduler and saw that
> it makes lot of mmap/munmap.
> For example during second 1376487042 it has called mmap 72 times with
> sizes 1052672, 2101248.
> Such lines are very interesting for me:
> 1376487042.167246 mmap(NULL, 1052672, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fe8f49bf000
> 1376487042.174690 munmap(0x7fe90c6ff000, 1052672) = 0
> erlang asked system for one megabyte of memory and after 7 milliseconds
> returned it back.
> To make it clear: erlyvideo is reading 700-800 megabytes per second from
> several hard drives, repack them a bit and sends to network via TCP sockets.
> It is possible to tell erlang that it can use 20GB or 30 GB or ram and
> don't return it back at all?
> I suppose that such often allocation/deallocation leads to high cpu usage
> of process kswapd0 that (perhaps) tries to do something with such big
> amount of freed pages.
> I have looked at erts_alloc.html page. Well, it is scary a bit, just like
> when I've looked into the airplane cabin. Should I change any options from
> this page or I shouldn't touch memory allocator at all?
The erts_alloc docs can be a bit intimidating.

The "mseg" allocator is the backend that makes the calls to mmap/munmap. 
It contains a cache of memory segments to reduce the number of 
mmap/munmap calls. Have you tried to increase the size of the mseg cache 

+MMmcs <amount>
    Max cached segments. The maximum number of memory segments stored in 
the memory segment cache. Valid range is 0-30. Default value is 10.

/Sverker, Erlang/OTP

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