Max RAM

[cognostechie]

I was wondering what is the maximum RAM that can/should be allotted to Cognos. I am referring to the 'Maximum memory..' property of the Cognos service in Cognos configuration. Our server has 32 GB.

[Suraj]

From guide:
    To reduce the startup time, memory footprint, and resources used, use the default setting of 768.

    To balance between fast startup time and quick operating speeds, type a value about 1.5 times the default value, such as 1152.

    To maximize operating speeds and if performance is more important than fast startup time, and if your computer has a lot of resources, type a value about double the default value, such as 1536.
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However, I've changed it up and down depending upon how the server is holding up.
If you see too many server not responding errors, try changing it and see if that eases it up.

I had few times where the services could not even start unless I backed down to 768.

[cognostechie]

Really ! So what's the point in having so much RAM on the server if Cognos cannot use more than 768 MB ? I wonder why it is designed like that !

[Suraj]

Really ! So what's the point in having so much RAM on the server if Cognos cannot use more than 768 MB ? I wonder why it is designed like that !

RAM on the server is not only for the JVM.
Everything that runs in the server such as reports, cache and many other processes use RAM.
If you make the Cognos memory too big, it can be slow and unstable opposite of expected results.

[cognostechie]

I understand but going by the documentation which says that if you want a faster startup time then use 768 MB and if you want better performance at the cost of increasing the start up time then make it 1536 MB. So what does this mean? That if you increase the value
in this setting then performance of the reports will be better? With that , if you increase it to 4 GB then it would be even better? Of course, if the server has 64 GB RAM, I am not going to put 32 GB in this setting because RAM is required for other things too but then the question is why do they say about 1536 MB only and not more than that? Also, what is the 'startup' time? Is it the time taken to start the Cognos service or time taken to start running the report?

I am trying to understand a little deeper instead of just accepting the documentation because I found in one instance that the documentation doesn't always get updated with the release of the new version. Is it possible that this piece was not updated for quite some time when the memory used to be in MBs and not GBs ? For servers in today's technology, why would it make much of a difference if you allotted 2 GB or 4 GB because what percentage is that of 64 GB? Of course, when the RAM used to be 4 GB, then 768 vs 1536 would have made a difference.

I had seen this setting in the documentation years ago and at that time it said the same thing '768 MB' . At that time, people used to have much less RAM on their server so why should 1536 still be considered as the max for this setting when it was the max recommended at that time ? How much is 'too much' ?

or maybe I just shouldn't bother about this and go by what the doc says?

[bdbits]

I am not a server admin these days, but those numbers are familiar from tuning JVMs in my last job. It is more good old JVM tuning than a Cognos-specific thing. You can find tons of info on JVM tuning, and you can google the various ways of monitoring JVMs to find optimal settings. From memory, the tradeoffs on memory allocations mostly have to do with making sure the Java memory management routines (e.g. garbage collection and heap management) occur often enough, but not too often. There is a point of diminishing returns, so bumping it to 4G is probably not going to help as the JVM will become inefficient. And as Suraj pointed out, Cognos will spin up plenty of other processes that utilize RAM, and I know there are additional tuning options for at least some of these.

As a rule of thumb, I usually go by whatever product documentation recommends unless I have some evidence there is a good reason to override it.

[cognostechie]

Just to share, it tests out fine uptil 6 GB and above that it does not accept it when it is tested. That because my server RAM is 64 GB. Anyway, I followed your suggestion and made it 1536 and seems to working fine.

[Suraj]

Just to share, it tests out fine uptil 6 GB and above that it does not accept it when it is tested. That because my server RAM is 64 GB. Anyway, I followed your suggestion and made it 1536 and seems to working fine.

Start time is for Cognos services to start and become available.
Cognos documentation will be always little behind as they have to work with what's universally/mostly used at the clients' sites, not what's the latest and greatest out there.

Never tried that high.
If 6GB runs and is stable, give it a shot but watch out for any issue actively.