[erlang-questions] ETS memory fragmentation after deleting data

Dániel Szoboszlay dszoboszlay@REDACTED
Mon Feb 18 14:06:44 CET 2019

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.

Thanks again for the insight, Sverker!


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
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