[erlang-questions] How can I release beam process memory?
Michael Truog
mjtruog@REDACTED
Thu Apr 6 20:11:25 CEST 2017
Hi Jack,
The simplest way to make an Erlang process automatically GC is to use a temporary Erlang process, since the death of the Erlang process will cause GC to occur soon after. It is common to avoid long-lived Erlang processes that create lots of temporary data quickly due to excessive memory consumption, and to refactor the long-lived process a temporary process is used to create the temporary data within (this matters at high request rates, though you may not see the problem with low request rates). You may also tune an Erlang process with fullsweep_after to cause GC to occur more frequently, but I see that as only performance tuning, not fixing a problem that should be fixed with a temporary Erlang process. There also is the erlang:garbage_collect function, though you shouldn't use that in production source code (source code that needs to be dependable as its expectations change).
Best Regards,
Michael
On 04/06/2017 07:41 AM, Jack Tang wrote:
> Hi Daniel
>
> Thank you for the detailed explanation and sharing the experiences. I focused on the performance of GC heavily before and ignored the allocation strategy.
>
> One more question, as you mentioned memory on eheap_alloc can be released by calling GC on every process. Right now I setup periodic gc process to free eheap. However can I set some process flag to make GC automatically?
>
> Thanks again!
>
> BR
> -Jack
>
> On Thu, Apr 6, 2017 at 7:52 PM, Dániel Szoboszlay <dszoboszlay@REDACTED <mailto:dszoboszlay@REDACTED>> wrote:
>
> Hi Jack,
>
> *tl;dr;* try starting Erlang with '+MEasbf'
>
> Long explanation:
>
> If the memory is used by the ets_alloc allocator, it can be either actively used by ETS tables (such as Mnesia disc_copies tables) or it can be lost due to memory fragmentation. I guess this is memory fragmentation in your case, but let's quickly rule out the other possibility first. (By the way, in either case, garbage collecting processes won't reclaim the memory. GC operates on eheap_alloc allocated memory.)
>
> So if erlang:memory(ets) is close to what recon reports as allocated, the memory is in use. Your system wrote a lot of data to ETS/Mnesia, and didn't remove it afterwards. Inspect the contents of the tables and figure out what was left there that shouldn't be there, which part of the application should have removed that data and why didn't it do its job properly.
>
> The other, more likely option is fragmentation. I also experienced that the default memory allocation strategy (aoffcbf = address order first fit carrier best fit) can perform poorly when you use a lot of memory. All address order first fit strategies will use heavily the multiblock carriers with the lowest memory addresses, and if you have many carriers, those placed higher in memory will have less and less chance to be used. In my particular case ets_alloc created a total of 150k multiblock carriers for storing ~1.2TB data in ETS tables. This resulted in ~100 GB unused memory being wasted in the high address carriers. You have a much smaller system, but depending on the usage patterns of your ETS data you can end up in a similar situation.
>
> You can check the number of carriers with erlang:system_info({allocator, ets_alloc}). It will print something like this:
> [{instance,0,
> [{versions,"0.9","3.0"},
> {options,[...]},
> {mbcs,[...
> {carriers,1,1,1}, %% <- number of multi block carriers = 1
> ...]},
> {sbcs,[...
> {carriers,0,0,0}, %% <- number of single block carriers = 0
> ...]},
> {calls,[...]}]},
> {instance,1,...
> Check theaw numbera across all your allocator instances. You will typically have very few single block carriers (unless you store huge records in ETS). In my experience, fragmentation correlates well with the number of carriers an allocator handles, and can become quite significant above ~10 carriers.
>
> So, If you have the same problem I described, I have some bad news and some good news. The bad news is that I don't know a way of forcing the VM to defragment memory and get rid of the waste. The good news is that the bf (best fit) allocation strategy (which used to be the default up to R16) performs much better when you have many carriers. You need to pass the '+MEas bf' command line argument to the VM to switch ets_alloc to bf strategy.
>
> Hope it helps,
> Daniel
>
> On Sat, 1 Apr 2017 at 05:36 Jack Tang <himars@REDACTED <mailto:himars@REDACTED>> wrote:
>
> After setting up erlang memory visualization, we find etc allocator does not release the memory during some historical datum are remove from mnesia tables. Can I release the memory on the fly rather than restart the mnesia application? Thanks!
>
>
>
>
> BR
>
> On Sun, Jan 15, 2017 at 4:47 AM, Dániel Szoboszlay <dszoboszlay@REDACTED <mailto:dszoboszlay@REDACTED>> wrote:
>
> Hi Jack,
>
> I guess the 9 GB is lost due to memory fragmentation. Erlang allocates memory in large chunks called carriers from the OS, then places the blocks your program actually needs on these carriers. A carrier can only be returned to the OS once all the blocks on it have been freed (and even then, the memory allocator may decide to keep it around for a while in case more memory is needed).
>
> You can check with recon_alloc <https://ferd.github.io/recon/recon_alloc.html> how much unused memory is lost due to fragmentation in the various allocators.
>
> The bad news is that you cannot defragment the carriers, and if the selected memory allocator strategy doesn't work well for your application, you cannot change it either without restarting the emulator.
>
> However, if the memory is wasted in the eheap_alloc, you may try to force a GC on all processes a couple of times. As the GC copies the memory, it will allocate new blocks and free up the old heap blocks. So there's a chance the allocators can compact the blocks together on fewer segments. But that's just a guess, it may or may not work at all.
>
> Cheers,
> Daniel
>
> On Sat, 14 Jan 2017 at 08:04 Jack Tang <himars@REDACTED <mailto:himars@REDACTED>> wrote:
>
> Hello list,
>
> I run one Erlang application on Debian server and today I find the beam process consumes around 35G memory by `top` command.
>
> ```
> KiB Mem: 99194912 total, 61682656 used, 37512252 free, 397380 buffers
> KiB Swap: 0 total, 0 used, 0 free. 18684864 cached Mem
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 11858 usr1 20 0 36.850g 0.032t 6220 S 73.5 34.4 8038:49 beam.smp
> ```
>
> I connect to the Erlang application using remote shell and find the mem-leaked supervisor tree and run gc on the whole tree. Code looks like blow:
>
> ```
> lists:foreach(fun(E) -> PId = element(2, E), erlang:garbage_collect(PId) end, supervisor:which_children(some_thing_sup)).
> ```
>
> and erlang:memory() decreases from 32G to 23G.
> ```
> [{total,22982011544},
> {processes,12884182336},
> {processes_used,12884170336},
> {system,10097829208},
> {atom,13828705},
> {atom_used,13796692},
> {binary,170530288},
> {code,16450626},
> {ets,9637717576}]
> ```
>
> However, when I input `top` command, the beam process still takes 35G memory. What can I do to release the 9G memory? Thanks
>
> BR
> -Jack
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> --
> Jack Tang
>
>
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> --
> Jack Tang
>
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