[erlang-questions] ETS memory fragmentation after deleting data

Lukas Larsson lukas@REDACTED
Mon Feb 18 14:15:15 CET 2019

On Mon, Feb 18, 2019 at 2:07 PM Dániel Szoboszlay <dszoboszlay@REDACTED>

> Table segments could indeed be responsible for keeping some carriers
> alive. I deleted 750k objects out of the initial 1M, so I had to hit the
> shrink limit at one point, but probably not for a second time. And
> overwriting the contents of the table with an equal number of objects
> cannot defragment table segment memory, since there's no need to reallocate
> those.
> I wish there was a function to force the compaction of an ETS table and
> maybe even defragment ETS allocators a bit. But I guess it wouldn't be
> useful for enough people to justify the development cost. It may be easier
> to simply restart the Erlang node after deleting a lot of data.

No need to restart the node, the same effect should be possible to achieve
by deleting the table and re-creating it.

> Thanks again for the insight, Sverker!
> Daniel
> On Tue, 12 Feb 2019 at 18:48 Sverker Eriksson <sverker@REDACTED> wrote:
>> I think the table segments are whats keeping the carriers  alive.
>> When a set,bag or duplicate_bag grows new table segments are allocated.
>> Each new segment contains 2048 hash buckets and the load limit for growth
>> is 100%.
>> This means for every 2048 object you insert a new segments i allocated.
>> The load limit for shrinking is 50%, so after inserting 1 miljon objects
>> you have to delete 0.5 miljon before the table starts to shrink and
>> segments are deallocated.
>> Increasing the shrink limit will reduce carrier fragmentation in your
>> case,
>> but it may also cost in performance from more frequent rehashing when
>> number of objects fluctuates.
>> The shrink limit is controlled by
>> in erts/emulator/beam/erl_db_hash.c
>> /Sverker
>> On lör, 2019-02-09 at 00:30 +0100, Dániel Szoboszlay wrote:
>> Hi Sverker,
>> Thanks for the tip, I changed my code in the gist to use erlang:system_info({allocator,
>> ets_alloc}) and the weirdest things disappeared. (Also, I intentionally
>> avoided storing binaries in the ETS table in this test, so the
>> binary_alloc couldn't play a role in the results.)
>> But now I see different "problems":
>>    - Deleting from the ETS table cannot free up any of the carriers. :(
>>    After deleting 75% of the objects I could regain 0 memory for the OS
>>    and the utilisation is down to a disappointing 25%.
>>    - Overwriting every object once with itself sometimes have no effect
>>    at all on the carrier size either. In this case a second round of
>>    overwrites are needed to free up carriers.
>>    - My memory compaction trick can now only achieve 50% utilisation. So
>>    the memory is still fragmented.
>>    - I tried to repeat the overwrite step a few more times, but once it
>>    reaches 50% utilisation it cannot improve on it any more.
>> My guess was that maybe carrier abandoning causes this problem. I tried
>> playing with +MEacul 0, some different +MEas settings and even with +MEramv
>> true, but neither of them helped.
>> So my new questions are:
>>    - What may be preventing my overwrite-with-self-compactor to go above
>>    50% carrier utilisation?
>>    - Is there any trick that would help me further reduce the
>>    fragmentation and get back to 90%+ utilisation after deleting a lot of
>>    objects from ETS?
>>    - Wouldn't ERTS benefit from some built-in memory defragmentator
>>    utility, at least for ets_alloc? (For example I don't think eheap_alloc
>>    would need it: the copying GC effectively performs defragmentation
>>    automatically. binary_alloc would also be a potential candidate, but it may
>>    be significantly harder to implement, and I guess most systems store less
>>    binary data than ETS data.)
>> Thanks,
>> Daniel
>> On Thu, 7 Feb 2019 at 22:25 Sverker Eriksson <sverker@REDACTED> wrote:
>> Hi Dániel
>> I looked at your test code and I think it can be the 'mbcs_pool' stats
>> that are missing.
>> They are returned as {mbcs_pool,[{blocks_size,0}]} without carriers_size
>> for some reason
>> by erlang:system_info({*allocator_sizes*,ets_alloc}).
>> Use erlang:system_info({*allocator*,ets_alloc}) to get mbcs_pool with
>> both block and carrier sizes.
>> Another thing that might confuse is that all binaries larger than 64
>> bytes will be stored in binary_alloc.
>> /Sverker
>> On tor, 2019-02-07 at 15:35 +0100, Dániel Szoboszlay wrote:
>> Hi,
>> I would like to understand some things about ETS memory fragmentation
>> after deleting data. My current (probably faulty) mental model of the issue
>> looks like this:
>>    - For every object in an ETS table a block is allocated on a carrier
>>    (typically a multi-block carrier, unless the object is huge).
>>    - Besides the objects themselves, the ETS table obviously needs some
>>    additional blocks too to describe the hash table data structure. The size
>>    of this data shall be small compared to the object data however (since ETS
>>    is not terribly space-inefficient), so I won't think about them any more.
>>    - If I delete some objects from an ETS table, the corresponding
>>    blocks are deallocated. However, the rest of the objects remain in their
>>    original location, so the carriers cannot be deallocated (unless all of
>>    their objects get deleted).
>>    - This implies that deleting a lot of data from ETS tables would lead
>>    to memory fragmentation.
>>    - Since there's no way to force ETS to rearrange the objects it
>>    already stores, the memory remains fragmented until subsequent updates to
>>    ETS tables fill the gaps with new objects.
>> I wrote a small test program (available here
>> <https://gist.github.com/dszoboszlay/921b26a57463ec1f5df1816a840a78aa>)
>> to verify my mental model. But it doesn't exactly behave as I expected.
>>    1. I create an ETS table and populate it with 1M objects, where each
>>    object is 1027 words large.
>>    I expect the total ETS memory use to be around 1M * 1027 * 8 bytes ~
>>    7835 MiB (the size of all other ETS tables on a newly started Erlang node
>>    is negligible).
>>    And indeed I see that the total block size is ~7881 MiB and the total
>>    carrier size is ~7885 MiB (99.95% utilisation).
>>    2. I delete 75% of the objects randomly.
>>    I expect the block size to go down by ~75% and the carrier size with
>>    some smaller value.
>>    In practice however the block size goes down by 87%, while the
>>    carrier size drops by 48% (resulting in a disappointing 25% utilisation).
>>    3. Finally, I try to defragment the memory by overwriting each object
>>    that was left in the table with itself.
>>    I expect this operation to have no effect on the block size, but
>>    close the gap between the block size and carrier size by compacting the
>>    blocks on fewer carriers.
>>    In practice however the block size goes up by 91%(!!!), while the
>>    carrier size comes down very close to this new block size (utilisation is
>>    back at 99.56%). All in all, compared to the initial state in step 1, both
>>    block and carrier size is down by 75%.
>> So here's the list of things I don't understand or know based on this
>> exercise:
>>    - How could the block size drop by 87% after deleting 75% of the data
>>    in step 2?
>>    - Why did overwriting each object with itself resulted in almost
>>    doubling the block size?
>>    - Would you consider running a select_replace to compact a table
>>    after deletions safe in production? E.g. doing it on a Mnesia table that's
>>    several GB-s in size and is actively used by Mnesia transactions. (I know
>>    the replace is atomic on each object, but how would a long running replace
>>    affect the execution time of other operations for example?)
>>    - Step 3 helped to reclaim unused memory, but it almost doubled the
>>    used memory (the block size). I don't know what caused this behaviour, but
>>    is there an operation that would achieve the opposite effect? That is,
>>    without altering the contents of the table reduce the block size by 45-50%?
>> Thanks,
>> Daniel
>> _______________________________________________
>> erlang-questions mailing listerlang-questions@REDACTED://erlang.org/mailman/listinfo/erlang-questions
>> _______________________________________________
>> erlang-questions mailing list
>> erlang-questions@REDACTED
>> http://erlang.org/mailman/listinfo/erlang-questions
>> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20190218/a70f630d/attachment.htm>

More information about the erlang-questions mailing list