Moodle in English: What is the correct MUC setting for running Moodle 2.4.4+ in a clustering environment?

What is the correct MUC setting for running Moodle 2.4.4+ in a clustering environment?

by CP Lau - Tuesday, July 2, 2013, 1:51 PM

I am performing load test on Moodle 2.4.4+ in a clustering environment (i.e. 1 LB, 5 Moodle web servers, 1 MySQL db server). However, the result is very very very poor if i follow the instruction of Moodle to put the cachedir in a shared folder and cachedir/langstring in a local folder. The MUC cache settings are also the default settings.

OpenU of UK also mentions the Moodle 2.4 slowness problem in http://planet.moodle.org/ and her suggestion is to use memcache for MUC and suggest "nine speed-ups we can make to Moodle 2.4 in the core Moodle code".

May i ask

1) where i should put the cache folder? a shared folder or local one?

2) how should i configure the cache store for request, session and application caches in Moodle MUC? Using memcache for all of them?

3) just setup 1 memcache server to serve 5 Moodle web servers?

Thanks a lot.

Re: What is the correct MUC setting for running Moodle 2.4.4+ in a clustering environment?

by Tim Hunt - Tuesday, July 2, 2013, 2:33 PM

1) if the cache folder is on disc, it must be on a shared disc.

but that is probably too slow, so you should change to something else, like memcache.

2) you need to think about this. You will note that some cache stores have properies like "data guarantee, key awareness" which memcache cannot offer, and sometimes these are required. At the OU we set

Under Known cache definitions:

Language string cache, Database meta information, Question definitions & HTML Purifier - cleaned content - change to use memcache.
Event invalidation - leave that as the default.

Under Stores used when no mapping is present:

Session - change to memcache
Application & Request - leave on the default.

3) memcache works best when it can use as much memory as possible, adn when the load is spread over many servers, so install it on all 5 web servers, and let it use all the spare memory.

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Re: What is the correct MUC setting for running Moodle 2.4.4+ in a clustering environment?

by CP Lau - Monday, July 15, 2013, 10:15 AM

Dear Tim, I have followed your instructions in setting up the MUC with memcache. The result of the load test then becomes good. Thank you very much.

However, i still have the following questions which need your valuable advice:

1) When i setup memcache in the page "Cache configuration", i need to restart the web server after each change. And i need to modify the configuration file directly in order to set up memcache for session cache. Is it normal?

2) Will Moodle performance deteriorate significantly if one of the memcache servers is crashed?

3) What is the best way to purge Moodle caches? By performing "Development/Purge all caches"?

4) It seems there is improvement in the table locking issue (pls refer to https://moodle.org/mod/forum/discuss.php?d=221444) of mdl_cache_text in v2.4.4+ when there are many concurrent updates. Would you still recommend to set Text cache lifetime to No for large site with many concurrent updates as i want to achieve even more concurrent updates?

5) If i perform "Purge all caches" before performing the load test, the total number of hits would become smaller, say by 12% than the test without purging all caches. Is it normal?

Thank you very much.

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Hardware and performance

What is the correct MUC setting for running Moodle 2.4.4+ in a clustering environment?