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<div>Hi Dániel </div>
<div><br>
</div>
<div>I looked at your test code and I think it can be the 'mbcs_pool' stats that are missing.</div>
<div><br>
</div>
<div>They are returned as {mbcs_pool,[{blocks_size,0}]} without carriers_size for some reason</div>
<div>by erlang:system_info({<b>allocator_sizes</b>,ets_alloc}).</div>
<div><br>
</div>
<div>Use erlang:system_info({<b>allocator</b>,ets_alloc}) to get mbcs_pool with both block and carrier sizes.</div>
<div><br>
</div>
<div>Another thing that might confuse is that all binaries larger than 64 bytes will be stored in binary_alloc.</div>
<div><br>
</div>
<div>/Sverker</div>
<div><br>
</div>
<div>On tor, 2019-02-07 at 15:35 +0100, Dániel Szoboszlay wrote:</div>
<blockquote type="cite">
<div dir="ltr">Hi,
<div><br>
</div>
<div>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:</div>
<div>
<ul>
<li>For every object in an ETS table a block is allocated on a carrier (typically a multi-block carrier, unless the object is huge).</li><li>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.</li><li>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).</li><li>This implies that deleting a lot of data from ETS tables would lead to memory fragmentation.</li><li>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.</li></ul>
<div>I wrote a small test program (<a href="https://gist.github.com/dszoboszlay/921b26a57463ec1f5df1816a840a78aa">available here</a>) to verify my mental model. But it doesn't exactly behave as I expected.</div>
</div>
<div>
<ol>
<li>I create an ETS table and populate it with 1M objects, where each object is 1027 words large.<br>
<br>
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).<br>
<br>
And indeed I see that the total block size is ~7881 MiB and the total carrier size is ~7885 MiB (99.95% utilisation).</li><li>I delete 75% of the objects randomly.<br>
<br>
I expect the block size to go down by ~75% and the carrier size with some smaller value.<br>
<br>
In practice however the block size goes down by 87%, while the carrier size drops by 48% (resulting in a disappointing 25% utilisation).<br>
</li><li>Finally, I try to defragment the memory by overwriting each object that was left in the table with itself.<br>
<br>
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.<br>
<br>
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%.</li></ol>
<div>So here's the list of things I don't understand or know based on this exercise:</div>
</div>
<div>
<ul>
<li>How could the block size drop by 87% after deleting 75% of the data in step 2?<br>
</li><li>Why did overwriting each object with itself resulted in almost doubling the block size?</li><li>Would you consider running a <font face="monospace">select_replace</font> 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?)</li><li>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%?</li></ul>
<div>Thanks,</div>
</div>
<div>Daniel</div>
</div>
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