On Thu, 1 Sep 2011 15:01:52 -0400
Brad Hinson <[email protected]> wrote:
> Hi James,
>
> It appears that in order to use hardware large page support,
> Linux must be running in LPAR mode. I can't find anything that
> says this is supported in z/VM. Hopefully someone can correct
> me if I'm wrong. I can confirm that on a z10 under z/VM 6.1
> I also do not see 'edat' in /proc/cpuinfo, so hugepage support
> is emulated in software.
You can use large pages in LPAR and under z/VM. In LPAR we have
"real" large pages if we have the edat facility. If there is no
edat facility or if we are running under z/VM we use large page
emulation.
There are two benefits to using hugepages:
1) The TLB pressure in reduced by using 1MB frames. To get this
benefit the edat facility is required since this needs the
large page segment table entries. No love here for z/VM.
2) The memory savings due to the reduced number of page tables.
There are two cases:
2a) under LPAR with edat the 1MB frames are directly referenced
by the segment table entry, the lowest page table level is
not allocated at all.
2b) under z/VM there is no edat facility and no large page
segment table entries. Here a single page table for the 1MB
frame is allocated which is shared by all users of the large
page.
The page table overhead to map 2GB of memory:
i) without large pages: 1 segment table, 2048 page tables
ii) with large page emulation: 1 segment table, 1 page table
iii) with edat large pages: 1 segment table
In numbers i) 4112 KB, ii) 18 KB, and iii) 16 KB. This number
is per process. If your database uses processes for its
transactions and maps large share memory areas the memory
savings quickly add up.
If you have e.g. 128 processes mapping 2GB you'll need for
case i) 514 MB, ii) 2.25 MB, and iii) 2 MB.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
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