Re: Hugepages - benefits / drawbacks

Krishna Manoharan wrote,on my timestamp of 7/10/2008 10:51 AM:

> 1. Huge pages are used for SGA only. Don't reserve more than you intend
> to allocate to SGA (SGA_MAX_SIZE). Otherwise, you are going to see heavy
> swapping.

That would be highly dependent on the implementation.

> 2. Huge pages cannot be paged out. They are locked in memory (similar to
> ISM on Solaris) and even if un-used, not freed back to the OS. No other
> application other than Oracle can use this memory.

If they cannot be paged out, how can they result in heavy swapping?
Like I said: dependent on implementation. Solaris is just one of many.

> 5. Huge pages are more efficient because of the bigger pagesize (2M)
> and also help reduce TLB misses (assuming you are having a system with
> more than 16GB Memory).

Actually, I've seen them be useful at much less than 16GB. Typical TLBs can cope well with 32-bit size memories, or in other words: 4GB. After that, they start to show locate speed issues: simply, way too many page entries. The hugepage size reduces the number of required entries in the TLB for a given memory size. With Red Hat, hugepages show CPU usage improvement after 4GB - assuming of course one has an SGA of matching size and a workload that can show problems.

-- 
Cheers
Nuno Souto
in sunny Sydney, Australia
dbvision_at_iinet.net.au
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