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docs: add note about PYTHONMALLOC for accurate jemalloc memory tracking (#17709) 

Added a note in the documentation suggesting that users may set
`PYTHONMALLOC=malloc` when using `jemalloc`. This allows jemalloc to
track memory usage more accurately by bypassing Python's internal
small-object allocator (`pymalloc`), helping to ensure that
`cache_autotuning` functions as expected.

This doc change aims to provide more clarity for users configuring
jemalloc with Synapse.


Based on:
4ac783549c/synapse/metrics/jemalloc.py (L198-L201)
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Henrique 2024-10-07 05:37:39 -03:00 committed by GitHub
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Add documentation note about PYTHONMALLOC for accurate jemalloc memory tracking. Contributed by @hensg.

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docs/usage/administration/admin_faq.md
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@ -255,6 +255,8 @@ line to `/etc/default/matrix-synapse`:
LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.2
*Note*: You may need to set `PYTHONMALLOC=malloc` to ensure that `jemalloc` can accurately calculate memory usage. By default, Python uses its internal small-object allocator, which may interfere with jemalloc's ability to track memory consumption correctly. This could prevent the [cache_autotuning](../configuration/config_documentation.md#caches-and-associated-values) feature from functioning as expected, as the Python allocator may not reach the memory threshold set by `max_cache_memory_usage`, thus not triggering the cache eviction process.
This made a significant difference on Python 2.7 - it's unclear how
much of an improvement it provides on Python 3.x.