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

Björn-Egil Dahlberg wallentin.dahlberg@REDACTED
Mon Aug 26 15:52:44 CEST 2013

As of R16 MBCs are allocated in fixed sizes (power of 2) and each size has
its own freelist. The region sizes for new MBCs grows according to
allocating pressure. SBCs are different. The freelist for SBCs is a
circular list (but conceptually a stack). Free SBCs are pushed to the
freelist (you will usually use the same size very soon, hence you want it
first). When allocing a SBC the segment allocator will search the freelist
for a segment with suitable size and if none is found it will then map a
new segment. The freelist is emptied at the end of the list and these slots
are unmapped periodically as well as when cache clearing occurs, i.e.
nearing end of memory.

The memory allocator is a fairly complex piece of machinery. It balances
speed, scalability and fragmentation and does this on multiple levels. You
want to get most of your allocations in the MBCs but for large heaps and
large binaries this is of course impossible.

There is no real downside here but memory tuning can be highly application
dependent and there is no real "one size fits all". My suggestion is to
tune your system and measure.

I do agree that there should be an option to pre-map/allocate the entire
space of allocatable memory for the runtime system. It would be better for
the segment allocator to handle the entire region itself rather the to rely
on the operating system. We will probably add an option for it sometime but
I don't know when, maybe for R17 but probably later.

// Björn-Egil, still on vacation.

2013/8/26 Peer Stritzinger <peerst@REDACTED>

> On 2013-08-20 15:04:41 +0000, Lukas Larsson said:
>  Hello Peer,
>> On Tue, Aug 20, 2013 at 12:43 PM, Peer Stritzinger <peerst@REDACTED>
>> wrote:
>> My favorite behaviour would be for Erlang to either grab all memory
>> available or at least a fixed amount on startup and hold on to it forever.
>>  Hopefully without fragmenting it too much (I know in case of binary heaps
>> that this isn't possible which is a bit disturbing to me).
>> Can this be achieved by the current ERTS with some parameter settings?
>> You could request that the main mbc (+M<S>mmbcs) should be preallocated
>> to a certain size, and this would kind of get the effect you are looking
>> for. However this is controlled on a per allocator level, so you have to
>> figure out which type of memory it is that you are using. It also will only
>> work for any mbcs allocated, all sbcs will still be allocated using
>> mmap/malloc. If you want to force only mbcs you could up the sbct to very
>> big, just remember to up lmbcs as well so that can fit all of your sbcs
>> into the mbc.
>> I do not recommend doing this, but it is possible.
> I see, probably time is better to just optimize the system malloc for
> erlangs use cases.
>  Which direction should I look if I want to add behaviour like this?
>> BTW:  next best thing is to malloc/free allways same size tiles this
>> would avoid outer fragmentation and could be sped up relatively easily.
>> The emulator already tries to do this to the best of its abilities. mbcs
>> mmap requests grow depending on how you setup lmbcs, smbcs and mbcgs.
>> sys_alloc (i.e. malloc) calls are always multiples of +Muycs.
> Having allocs multiples of a fixed number helps against small scale
> fragmentation, on the larger scale if there is no memory mapping there is
> still possibility of fragmentation with this scheme.
>  You could set smbcs to the same as lmbcs if you want to, this makes it so
>> that all mbcs have that same size.
> I'l try this out, any downsides of doing this?
> This would remove all fragmentation issues and malloc/free could be sped
> up by just having a special free list for these fixed size tiles.
> Cheers
> -- Peer
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