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Physical memory usages vs SWAP/Virtual memory Spaces

Former Member
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Hello experts,

I need some expertise between physical memory usage and swap memory usage on Windows with SAP. Currently we have 32 GB physical memory hardware, with 8 processors. We also SWAP/Virtual memory set to initial 21 GB and maximum 30 GB.

Our instance profile PHYS_MEMSIZE = 20 GB, we are using Windows Zero Administration concept

Here is my question, as soon as we bring the SAP instance up, the PF usage get bumped to around 27 GB. And it is using the Virtual memory right away. I would expect the SAP to use the physical memory then when it only runs out of Physical memory it should be using the Virtual memory. Not sure what is exactly SAP Architecture this? Any thoughts anyone?

if I look my physical memory usage out of 32 GB, it only using like 5 GB. When does SAP uses the Physical memory?

If anyone can clarify this for me, it would be much helpful.

Thanks in advance.

Kumar

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Answers (1)

Answers (1)

markus_doehr2
Active Contributor
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This is a known issue with the Windows 2003 server family. You can try to install the hotfix:

Note 1009297 - Windows Server 2003 Family: High Paging Rates

Markus

Former Member
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Markus, I was really aware of the that note, however even after applying that note, no changes in the High Paging rate. Could you please help me understand when SAP really uses the physical memory, and why is it using the Virtual memory when the system starts up.....instead of the physical memory. Please advice.

Thanks

Kumar

markus_doehr2
Active Contributor
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SAP (and many other software products) always work with virtual memory, that's the pupose of having swap

During system start, SAP "sees" all memory - means your physical memory plus the swap space; in your case, it "sees" 43 GB of space. The application does not distinguish between "physical" and "virtual", it just finds out "Oh - there is 43 GB of memory available" (roughly spoken).

You have to take into account:

- the database also uses memory, depending on which database you use it's allocated on database start or subsequently up to the point configured. If you use the sapinst default, it will use half of your memory (so 16 GB)

- PHYS_MEMSIZE is the shared memory available for all workprocesses. It does not include your buffers (what you see in ST02/Buffers)

- the OS itself needs a certain amount of memory to work properly

How big is your em/initial_size_MB (you can check with RZ11).

Markus

Former Member
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Thanks Markus,

our em/initial_size_MB=20935

In our case, Database sits on the different host, since we clustered the systems. We have 2 application servers. One application sits with database host, which has more higher paging rate than the application servevr sits alone. If I bring SAP instance that is sitting on OWN server, this paging also very high. It has 30 GB Swap memroy. So total we have 60 GB total memory (Virtual + Physical). When I start the SAP instance page rate goes upto 30 GB right away. If I look at my total physical memory usage....like 5 GB out of 32 GB.

and Virtual memory being used like 20GB out 32 GB.....

Question I have been asked by server team, why does SAP NOT utilizing the Physical memory? why is it using the Virtual memory to run the application server? At one point SAP will be using the physical memory as well, my question is it part of Extended memory....when Extended memory % goes up...Is physical memory being used? ....To be honest I am bit of confused....

If you have help me understand, that would be really nice.

Thanks in advance.

Kumar

markus_doehr2
Active Contributor
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> When I start the SAP instance page rate goes upto 30 GB right away.

Where do you see that?

> If I look at my total physical memory usage....like 5 GB out of 32 GB. and Virtual memory being used like 20GB out 32 GB.....

Yes - because the system allocates the memory - but does not USE it at that moment.

Shared memory segments can't increase their sizes once they are built so the system allocates them but they are "empty".

The actual memory usage you can see in ST02 under "SAP memory" - there you see under "curr. use" the memory, that is actually used by that instance.

> Question I have been asked by server team, why does SAP NOT utilizing the Physical memory?

it does.

Markus

Former Member
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Not page rate, but PF Usage goes up to like 30 GB, I can see this via Windows Task Manager Monitoring.

Yes - because the system allocates the memory - but does not USE it at that moment.

I understand that, it allocates the memory but it does not use it right away....but why is it allocating from Virtual memory when you have like 25+ GB Physical Memory available? Like you said....SAP see that as one big Memory pool (virtual+physical), it would make more sense for me if that at least get split, like 50% from Virtual and 50% from Physical....but I just see that it uses almost all the virtual memory to allocate for that instance when the SAP Instance started and leave the 90% of the physical memory to hang around......

When you say shared memory segments, is allocated via PHYS_MEMSIZE right?

Here is my actual memory usage:

Roll memory     Dialog session   kB      2.930
                Nondialog sess.  kB      2.930
                Available        kB    262.144
                in shared memory kB    262.144
                on disk          kB          0
                Used             kB        395
                Maximum used     kB      6.816
Paging memory   Session buffer   kB      1.200
                Available        kB    262.144
                in shared memory kB    131.072
                on disk          kB    131.072
                Used             kB         22
                Maximum used     kB     97.008
Extended memory Dialog session   kB  1.953.125
                Nondialog sess.  kB  1.953.125
                Available        kB 20.127.744
                Used             kB    573.440
                Maximum used     kB  3.559.424
Heap memory     Dialog session   kB  1.953.125
                Nondialog sess.  kB          0
                Used             kB          0
                Maximum used     kB          0

markus_doehr2
Active Contributor
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> I understand that, it allocates the memory but it does not use it right away....but why is it allocating from Virtual memory when you have like 25+ GB Physical Memory available? Like you said....SAP see that as one big Memory pool (virtual+physical), it would make more sense for me if that at least get split, like 50% from Virtual and 50% from Physical....but I just see that it uses almost all the virtual memory to allocate for that instance when the SAP Instance started and leave the 90% of the physical memory to hang around......

Why this is done like this is explained here:

http://help.sap.com/saphelp_nw70/helpdata/EN/84/7ed384d81f11d188be0000e83539c3/frameset.htm

> When you say shared memory segments, is allocated via PHYS_MEMSIZE right?

Also - and all the buffers.

You can try: (as <sid>adm):

sappfpar check pf=<your-instance-profile>

Markus

Former Member
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Hi Kumar, have a look on this note: 1518419. It does not solve Your problem direct - but there You'll have the official SAP guideline for pagefile and virtual memory use. Actually there's a nice PDF document attached to the note explaining different scenarios.

However, I'm having the exact same issue; a system not utilizing the physical memory before swaping (w2k3). We have virtualized our application servers, and the specific notes about virtualization says that fx. memory overcomitment are not allowed in production. Hence, our server team also pointing out that we do not make use of all the allocated RAM for our machines.

Therefore I think it's a very relevant question You ask. Hope to find the answer soon.

Best regards Kurt