How to set physical/logical pagesizes?

[Tim Medford]

Is it correct that you have to set the physical pagesize at cold install time? (assuming you want something other than the default of 4k)??

I was in the middle of doing a cold install of HPUX 11.0 a few weeks back. I couldn't figure out how to change this setting? I actually called the HP support line and the engineer told me to just finish the install, it was a kernel parameter.

Then later when I tried finding the kernel parameter and couldn't I called back and a different engineer said I had to have done it at install time! Arggg!!!

Thanks in advance,
Tim

[James Quade]

Try vps_pagesize in sam???

[Bruce Regittko_1]

Hi,

The kernel parameter is vps_pagesize. It can be adjusted from within sam but a reboot will be necessary for the change to take effect.

See http://docs.hp.com//hpux/onlinedocs/os/KCparam.PagingParamsOverview.html for more information on this and other virtual memory parameters.

--Bruce

[Tim Medford]

Thanks for the replies, but I don't think is what I'm looking for. The vps_pagesize seems to be some kind of virtual segment size.

When you issue a 'dmesg' command, the last thing reported is the memory information. Even after changing the vps_pagesize to 8 and rebooting, it still reports:

physical page size = 4096 bytes

Thanks.

[Andy Monks · Accepted solution]

The hp-ux kernel uses a 4k page size. At 11, the kernel will use superpages when it wants/needs to for kernel data. You will always see 4k pages used at boot time.

vps_pagesize allow you to change the default for use processes system wide. You can also do it on a executable by executable using chatr (which I think is better), as some applications run better when tuned and others don't. It also depends a lot on exactly what hp machine your running it on.

[Tim Medford]

Thanks Andy. I will be using HPUX 11.0 on a brand new L3000 server.

So in this scenario you are telling me it's not possible to configure the actual HPUX pagesize above 4k? And that vps_pagesize would be the only way to adjust the pagesize?

Thanks

[rick jones]

Indeed, HP-UX continues to use a physical page size of 4096 bytes.

However, the PA 2.0 processors and their TLB's support variable virtual page sizes in 4x multiples of 4096 - 4K 16K, 64K, 256K, 1M, 256M. at some point, one or more of the CPU's allows 1GB virtual pages.

You can make a virtual page size change system wide with vps_pagesize and vps_ceiling. or, you can alter individual applications with the chatr(1m) command. There is also a vps parm that limits the maximum size one can get with a chatr'd binary.

I have found that if I increase vps_pagesize it will occasionally induce stack size limit errors. this may be related to my having seen places where folks tune maxssiz at the same time as vps_pagesize.

As for values to select, that is something of an art. you should be especially careful if your app/system does a lot of process creation and destruction. if the page size selected is too large, _and_ the processes are coming and going with frequency, the kernel will spend a great deal of time creating and destroying virtual address regions instead of caching them for reuse. if the processes are long-lived, this is not an issue. i forget what the magic value is - something beyond 32K I think.

[Andy Monks]

Basicall Tim that's it. The kernel always tried to create a larger page when it can (depending on what it's being asked for). This is called a super page.

All the vps_pagesize (and other parameters do), is allow you to limit a bit what applications do.

And as I said before, it's better changing them individually using chatr and measuring the performance, rather than globably attempting to do something and killing everything.

Not all applications perform faster with larger page sizes. It's also very cpu dependant.