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Add detail to cache_autotuning config option documentation #12776

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May 19, 2022
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Add detail to cache_autotuning config option documentation #12776

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22c3b3f
fix autotune documentation
H-Shay May 18, 2022
2368478
newsfragment
H-Shay May 18, 2022
8a31f33
requested changes
H-Shay May 18, 2022
37464ad
update memory size instructions
H-Shay May 18, 2022
ea6cc6e
Update docs/usage/configuration/config_documentation.md
H-Shay May 18, 2022
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2 changes: 2 additions & 0 deletions changelog.d/12776.doc
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@@ -0,0 +1,2 @@
Add info to documentation of config option `cache_autotuning`.

12 changes: 8 additions & 4 deletions docs/usage/configuration/config_documentation.md
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Expand Up @@ -1122,14 +1122,18 @@ Caching can be configured through the following sub-options:
* `cache_autotuning` and its sub-options `max_cache_memory_usage`, `target_cache_memory_usage`, and
`min_cache_ttl` work in conjunction with each other to maintain a balance between cache memory
usage and cache entry availability. You must be using [jemalloc](https://github.com/matrix-org/synapse#help-synapse-is-slow-and-eats-all-my-ramcpu)
to utilize this option, and all three of the options must be specified for this feature to work.
to utilize this option, and all three of the options must be specified for this feature to work. This option
defaults to off, enable it by providing values for the sub-options listed below. Please note that the feature will not work
and may cause unstable behavior (such as excessive empyting of caches) if all of the values are not provided.
When providing values for memory usage use M for MB and K for KB.
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Is this line necessary after #12777?

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I think it makes sense from a user-friendly standpoint, just in case someone was linked to this option or hadn't looked over the full manual. I did change it to say KiB and MiB and linked the comment to the config convention section for completeness.

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That makes sense -- my only concern is if we end up supporting GiB in the future. Perhaps this line should point to the size definition section instead?

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Great point, I've updated it, let me know what you think.

* `max_cache_memory_usage` sets a ceiling on how much memory the cache can use before caches begin to be continuously evicted.
They will continue to be evicted until the memory usage drops below the `target_memory_usage`, set in
the flag below, or until the `min_cache_ttl` is hit.
* `target_memory_usage` sets a rough target for the desired memory usage of the caches.
the setting below, or until the `min_cache_ttl` is hit. There is no default value for this option.
* `target_memory_usage` sets a rough target for the desired memory usage of the caches. There is no default value
for this option.
* `min_cache_ttl` sets a limit under which newer cache entries are not evicted and is only applied when
caches are actively being evicted/`max_cache_memory_usage` has been exceeded. This is to protect hot caches
from being emptied while Synapse is evicting due to memory.
from being emptied while Synapse is evicting due to memory. There is no default for this option.

Example configuration:
```yaml
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