fix: infinite growth of cache when auto eviction is disabled #34210

regisb · 2024-02-08T09:53:31Z

Description

The course structure cache keys were previously set with an explicit timeout of None which was overriding the cache default timeout of 2h. This was OK in environments where the cache is configured with a maximum memory limit and an automatic key eviction strategy. But in others (such as Tutor), the course structure cache could grow infinitely.

It was agreed that course structure cache keys should be long-lived but should respect the default cache structure timeout. Thus, we set here the TTL to 1 week.

We can also configure Tutor to use a cache eviction policy. But that means we need to set a maxmemory value in Redis. It's not possible to set a value that will be appropriate for everyone:

if it's higher than the total memory (e.g: in small instances), server will crash before the cache is filled.
if it's too low (e.g: in large instances), the whole platform will abruptly slow down as many cache entries are suddenly evicted.

That question of whether Tutor should define a cache eviction policy is still under discussion, but it can be resolved independently of this change.

Supporting information

See discussion here: overhangio/tutor#984

Testing instructions

Please provide detailed step-by-step instructions for testing this change.

Deadline

ASAP. We would like to backport this change in Tutor.

openedx-webhooks · 2024-02-08T09:53:36Z

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See the discussion here: #984 And the upstream PR here: openedx/edx-platform#34210 The tl;dr is that the Redis course structure cache was growing without bounds. While the upstream fix should resolve that issue, we decided that Tutor should have a maxmemory limit and an eviction policy set for operational safety.

See the discussion here: #984 And the upstream PR here: openedx/edx-platform#34210 The tl;dr is that the Redis course structure cache was growing without bounds. While the upstream fix should resolve that issue, we decided that Tutor should have a maxmemory limit and an eviction policy set for operational safety. To manually expire existing keys, run: tutor local run cms ./manage.py cms shell -c "from django.core.cache import caches; c = caches['course_structure_cache']; [c.expire(key, 604800) for key in c.keys('*')]"

ormsbee · 2024-02-09T23:50:22Z

@robrap: I wanted to bring this to your attention. I don't think it will have a negative impact on you folks, but I'd like to go over your config on Monday if you have time.

robrap · 2024-02-12T21:40:28Z

@ormsbee: I got your message, but haven’t gotten to this. Let’s coordinate in Slack.

ormsbee · 2024-02-12T21:57:08Z

@robrap: Scheduled the meeting for tomorrow. Thank you!

See the discussion here: #984 And the upstream PR here: openedx/edx-platform#34210 The tl;dr is that the Redis course structure cache was growing without bounds. While the upstream fix should resolve that issue, we decided that Tutor should have a maxmemory limit and an eviction policy set for operational safety. Thus, Redis now has a 4gb maxmemory. If you need more memory on your instance, you should implement the "redis-conf" patch. To manually expire existing keys, run: tutor local run cms ./manage.py cms shell -c "from django.core.cache import caches; c = caches['course_structure_cache']; [c.expire(key, 604800) for key in c.keys('*')]"

regisb · 2024-02-13T09:10:29Z

Thank you for being on top of this Dave & Robert :)

robrap · 2024-02-13T17:27:05Z

[inform] I'm in the midst of updating on our end, and will let you know when this is complete.
[request] It seems like this might warrant an ! in the commit message and a backward incompatible footnote that let's someone know that if they wish to maintain the old behavior, they will need an explicit cache setting for course_structure_cache and that its TIMEOUT setting needs to be set to None.

xmodule/modulestore/split_mongo/mongo_connection.py

robrap · 2024-02-13T18:23:34Z

@regisb @ormsbee: My setting changes have been deployed. I'm not sure if you want to delay this by a day, but I guess it can go whenever you wish.

regisb · 2024-02-14T08:38:50Z

I have updated the commit message to include backward-compatible instructions, as well as the ! breaking change prefix.

See discussion here: overhangio/tutor#984 This is a breaking change that will explicitely set the timeout of course structure cache entries to 1 week, instead of being unlimited. If you wish to revert to the former behaviour, you should set `course_structure_cache["TIMEOUT"] = None`. The course structure cache keys were previously set with an explicit timeout of `None` which was overriding the cache default timeout of 2h. This was OK in environments where the cache is configured with a maximum memory limit and an automatic key eviction strategy. But in others (such as Tutor), the course structure cache could grow infinitely. It was agreed that course structure cache keys should be long-lived but should respect the default cache structure timeout. Thus, we set here the TTL to 1 week. We can also configure Tutor to use a cache eviction policy. But that means we need to set a `maxmemory` value in Redis. It's not possible to set a value that will be appropriate for everyone: - if it's higher than the total memory (e.g: in small instances), server will crash before the cache is filled. - if it's too low (e.g: in large instances), the whole platform will abruptly slow down as many cache entries are suddenly evicted. That question of whether Tutor should define a cache eviction policy is still under discussion, but it can be resolved independently of this change.

openedx-webhooks · 2024-02-14T13:28:40Z

@regisb 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.

edx-pipeline-bot · 2024-02-14T14:45:27Z

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

edx-pipeline-bot · 2024-02-14T15:03:16Z

2U Release Notice: This PR has been deployed to the edX production environment.

edx-pipeline-bot · 2024-02-14T15:03:17Z

2U Release Notice: This PR has been deployed to the edX production environment.

See the discussion here: #984 And the upstream PR here: openedx/edx-platform#34210 The tl;dr is that the Redis course structure cache was growing without bounds. While the upstream fix should resolve that issue, we decided that Tutor should have a maxmemory limit and an eviction policy set for operational safety. Thus, Redis now has a 4gb maxmemory. If you need more memory on your instance, you should implement the "redis-conf" patch. To manually expire existing keys, run: tutor local run cms ./manage.py cms shell -c "from django.core.cache import caches; c = caches['course_structure_cache']; [c.expire(key, 604800) for key in c.keys('*')]"

openedx-webhooks added the open-source-contribution PR author is not from Axim or 2U label Feb 8, 2024

regisb mentioned this pull request Feb 8, 2024

fix: allow to setup redis maxmemory overhangio/tutor#984

Closed

regisb mentioned this pull request Feb 8, 2024

fix: infinite cache growth overhangio/tutor#1001

Merged

robrap reviewed Feb 13, 2024

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xmodule/modulestore/split_mongo/mongo_connection.py Show resolved Hide resolved

regisb force-pushed the overhangio/course-structure-cache-timeout branch from 78185f6 to dfa16b3 Compare February 14, 2024 08:38

regisb force-pushed the overhangio/course-structure-cache-timeout branch from dfa16b3 to ad201cd Compare February 14, 2024 08:39

ormsbee approved these changes Feb 14, 2024

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ormsbee merged commit 4daf452 into openedx:master Feb 14, 2024
46 checks passed

regisb deleted the overhangio/course-structure-cache-timeout branch February 19, 2024 08:09

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fix: infinite growth of cache when auto eviction is disabled #34210

fix: infinite growth of cache when auto eviction is disabled #34210

regisb commented Feb 8, 2024 •

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ormsbee commented Feb 9, 2024 •

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robrap commented Feb 12, 2024

ormsbee commented Feb 12, 2024

regisb commented Feb 13, 2024

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edx-pipeline-bot commented Feb 14, 2024

fix: infinite growth of cache when auto eviction is disabled #34210

fix: infinite growth of cache when auto eviction is disabled #34210

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regisb commented Feb 8, 2024 • edited Loading

Description

Supporting information

Testing instructions

Deadline

openedx-webhooks commented Feb 8, 2024

ormsbee commented Feb 9, 2024 • edited Loading

robrap commented Feb 12, 2024

ormsbee commented Feb 12, 2024

regisb commented Feb 13, 2024

robrap commented Feb 13, 2024

robrap commented Feb 13, 2024

regisb commented Feb 14, 2024

openedx-webhooks commented Feb 14, 2024

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regisb commented Feb 8, 2024 •

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ormsbee commented Feb 9, 2024 •

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