Resident memory and used memory large gap

[Former Member]

Hi Team,

When we check from studio, resident memory vs used memory gap is very high.

Is there a way to do some flush or garbage cleanup and bring down the usage.

Hana version is :

HDB version info:

  version:             1.00.82.00.394270

Will put in some more information.

[Former Member]

A little bit off subject, but a question popped up into my head:

So memory manager requests more memory from OS if there is no more memory in HANA pool. This will continue until a "predefined allocation limit" (I guess the global_allocation_limit). So, I just wonder, what would happen if our resident memory is let's say 200 GB, and we lower the allocation limit to 150 GB - what would the monitors show? (yeap, we  can bring the allocation limit back to what it was right after as it's dynamically switchable).

[lucas_oliveira]

Hi,

Just to backup what Liz said, here's an SAP KBA describing your scenario:

2081473 - High Memory Useage: Resident Memory

BRs,

Lucas de Oliveira

[Former Member]

Hi Lucas,

I think this topic has been raised several times earlier.

Like in the below thread.

https://scn.sap.com/thread/3424524

As you are aware OS tools will monitor from a operating system perspective which would be the resident memory. There is no mechanism as such to reduce the resident memory usage like the high water mark in DB6 .

System is would be monitored by the DBA team and the operating system administrators team. They wouldnt understand that the used memory is low but the resident memory is high.

So is there a mechanism to reduce the resident memory usage or bring down especially in cases when there is a large gap.

[lucas_oliveira]

Hello,

Yes you're right. It has been asked before. SAP has a Note about it and the administration guide has info regarding it as mentioned in thread you found. So, what's missing?

BRs,

Lucas de Oliveira

[Former Member]

Hi Lucas,

As i said earlier, want to bring it down. Something like we have HWM high water mark in DB6 is it possible?

So would like to know to more from you about the same.

[lucas_oliveira]

Hello,

In short: there's no such feature in HANA afaik. I understand there's no need for such feature. Documentation does not specify or imply anything on that regard, because it is acceptable to have resident memo utilization  > hana memo utilization.

Look at what the official admin guide says:

[...]
Resident Memory
Resident memory is the amount of physical memory that is actually being used from the perspective of the operating system.
It is possible that the Used Memory value is lower than the Database Resident value if SAP HANA returns memory back to its memory pool (for example, after a temporary computation) and does not inform the operating system. This is normal.
[...]

[...]
Once a temporary computation completes or a table is dropped, the freed memory is returned to the memory manager, which recycles it to its pool without informing the operating system. Therefore, from SAP HANA's perspective, the amount of used memory shrinks, but the processes' virtual and resident memory sizes are not affected. This creates a situation where the used memory value may shrink to below the size of SAP HANA's resident memory. This is normal.
[...]

So unless you're using other tools that might add up to the OS memory utilization, everything should be fine. No need to try limiting resident memory utilization.

BRs,

Lucas de Oliveira

[Former Member]

Hi Lucas,

We can completely understand that. But you have to understand. When you are in a corporate setup you will have OS specific tools which monitor the system as well.

So should we increase the thresholds then?

[lucas_oliveira]

Hello,

I completely understand what you're saying. You can use those under knowing the information pre-requisites contained in SAP Note - 1730929 - Using external tools in an SAP HANA appliance.

Notice that SAP/HW partner can ask you to deactivate external tools in case you have an issue and needs support. Why not trying that and checking if indeed the external tools are the one to blame?

If you need any further help I suggest you to bring more detailed information.

BRs,

Lucas de Oliveira

[Former Member]

Could you please describe why exactly you want to do that? It's normal to have used memory below resident memory.

[Former Member]

Hi Liz,

Thanks for your response.

I am aware of the large gap between used memory and resident memory but the difference cannot be ten folds.

When the used memory is 200 GB and resident memory is around 1900GB. I am not saying its a problem but i am saying is that from operating system wise its still very high usage.

SAP might have given the flexibility to lower the resident memory usage especially when the difference is very high.

[Former Member]

I don't think you should worry about the gap though - it's not the problem, it simply means you have a lot of memory that can be used later.

The big gap only means that at some point you had really high usage going on. So I would concentrate on identifying WHAT caused the gap rather then how to shrink it.

[Former Member]

Hi Liz,

Good Day!

If you check my response. HANA DB OS would be monitored by OS tools. We would be getting uncessary alerts. So dynamically reducing the resident memory would help a lot.

What caused the gap is definitely important but right now the priority is to get the resident memory shrink and give back atleast some part of memory back.