Re: Hugepages - benefits / drawbacks
Date: Tue, 07 Oct 2008 17:38:35 -0500
I believe this was obsoleted in Oracle 10.2, which is why you don't see it do anything.
The idea behind the parameter (_use_ism_for_pga) was that Solaris only allowed large pages for ISM segments prior to Solaris 9 and in Solaris 9, using large pages for non-ISM segments required manual intervention.
In Solaris 10, large pages became usable for many different kinds of memory depending on the allocations for each. Thus it became possible to use large pages for the PGA without having to put it in ISM.
But it still will depend on how the PGA is allocated. With 10.2.0.3, you can specify the "_realfree_heap_pagesize_hint = 4194304" and it will use 4 MB pages for the heap. These won't be ISM, but using "pmap -xs" will show the segments with a pagesize of 4M (assuming you're on SPARC).
The benefits of this will depend entirely on how your application uses the PGA. With large hash or sort areas needed, large pages can help performance significantly. For other applications, the benefit may be minimal.
Roman Podshivalov wrote, On 10/07/08 17:19:
> Not a problem at all.
> I tested this with various statements and verified pga allocation via
> v$sesstat view. Still I'm failing to understand the following logic. If
> something is using ISM that means something is somehow involved with
> shared memory segments, because ISM is nothing else but a different way
> of controlling and allocating shared segments. If any operation creates
> shared memory segment with or without ISM I should be able to observe it
> via ipcs command output. So far under Solaris ipcs output matches with
> allocated SGA, nothing else. And segment sizes are pretty static. Please
> point me out where this logic is failing.