Calculating working set size

[Chris Hoover]

Can anyone shed some light on how to calculate the working set size for a given mongodb server?  The mongo docs mention that size vs memory as a sharding consideration, but as far as I can see, there is no mention of how to calculate that number.

Thanks!

Chris

[Sam Millman]

There is no mention how to because there is no way to in a generic manner.

The working set is decided by how much data you need within, normally, a 10 min interval. That depends on how long you want caching to work here, but 10 mins is normally a good estimate.

So you have to basically use your queries and judge, with the current traffic to your application how much work it makes for MongoDB in a time frame.

Bad working sets can normally be judged by excessive memory usage, IO thrashing and general misbehaivour of the server when using MongoDB so you can use that as a measure to know when you should maybe look into how much working set you really need.

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[Wojons Tech]

I agree with Sammaya, to find your working set is not plug this number in here plug this number there run the math and it comes out. Your really going to need to see what your system is doing lets say you have a message board or blog. You may want to calulcate your working set as the amount of fourm topics and posts that have taken place in the last 24 hours, If you have a social based app you may want to calulcate the amount of ram you need for 1 hour worth of users. the best way I come up with my working set is to come up with some sort of SLA, and if your finding you cant keep to your SLA because of page faults due to lack of memory then you will need to add more memory.