11.23 Does pseudo swap compete with the dynamic buffer cache?

[Rodney Myers]

HP-UX 11.23 Itanium

There is a lot of disk I/O activity on our system, and there is a relationship between the size of the buffer cache and overall performance. Larger buffer cache = better overall performance. The buffer cache is dynamic on this system, i.e. dbc_min_pct is not equal to dbc_max_pct. Also swapmem_on is 1.

The machine has 12 GB of RAM. There is also 12GB of device swap configured at the moment.

Outside of unusual circumstances, there is not any device (disk) paging activity. However, there is enough memory pressure that the buffer cache does not get close to dbc_max_pct.

We have plenty of free disk space on this machine. I can create sufficient device swap, and set swapmem_on to 0. Will this have any effect on the size of the dynamic buffer cache? The goal is to move the memory reservation to the device swap, and use the RAM for buffer cache instead of pseudo swap.

I need opinions from those who know more about HP-UX internals than I do.

What do you think? Is this idea worth a test?

[A. Clay Stephenson]

In your case, since device swap >= physical memory, pseudoswap serves no purpose. It is really intended for boxes which have less swap than physical memory --- and it really isn't swap at all. It's just kernel bookkeeping. In your case, it will certainly do no harm to turn off pseudoswap thogh I don't expect any performance gains either.

The memory reservation doesn't really move either; again, it's kernel arithmatic. The space is still reserved. Bear in mind that you could have a huge buffer cache and still have very high physical i/o rates. It depends upon how much of your data is being accessed multiple times and how your overall data volumes compare to buffer cache size.

[Rodney Myers]

Thanks Clay. I understand that a large buffer cache does not necessarily mean low disk activity.

Are you saying that you expect swapmem_on = 0 to have no impact on the size of the dynamic buffer cache?

What are the disadvantages to swapmem_on = 0 for the system I described?

[whiteknight]

Rodney,

Refer to this link http://h21007.www2.hp.com/portal/download/files/unprot/devresource/Docs/TechPapers/UXPerfCookBook.pdf

for explanation why you need to turn on swapmemon=1

WK