[erlang-questions] ETS memory fragmentation after deleting data

Dániel Szoboszlay dszoboszlay@REDACTED
Sat Feb 9 00:30:43 CET 2019

Hi Sverker,

Thanks for the tip, I changed my code in the gist to use
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.)


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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