Hello,
[...]
Comparing SF68k/SF15k with Niagara is problematic. The broken MMU design in the US3/4 CPU models used in these machines is not able to use a significant amount of 64k pages. If you still got a small performance win there then this would prove that an all-64k kernel has significant performance advanges over the stock version with it's 8k "dwarf page" size.
Please explain what you mean by "broken". If you mean US-III, I would
agree, as this chip only supported 8K pages in the "T8" (the only big 512
entry) TLB. 64K performance was, well, problematic on that machine because
all the entries fight for the fully associative T16 TLB which only has
around 9 available unlocked entries. Not a pretty picture!
For this reason the autoMPSS code is turned off on US-III platforms. We
tried it, and it was miserable for some apps.
On the other hand, US-III+, US- IIIi, and later chips support programming
both large 512-entry data TLBs so there is no loss of associativity nor
capacity, so 64K pages work great within the DTLB. The performance
analysis was, as I recall, done mostly using US-III+.
The instruction TLB on some of these chips only supports 8K, but that has
a straightforward workaround in the form of TTE synthesis/replication.
US-IV+ supports 8K or 64K in the ITLB so you're set there.
The fact that one of the dual large TLBs is always "hard wired" to 8K in
Solaris today on US-III+/IV/IV+ is an artifact of the implementation and
doesn't reflect any limitation of the hardware.
Could Sun get the project code released into Opensolaris, please? I agree with both David
Miller and Roland Mainz that a kernel which uses 64k pages by default will have
significant performance advantages over the kernel which uses "dwarfpages". The
8k page size is a significant limitation.
I'm sure the archives are lying around somewhere.
As I stated earlier, I'd be interested in a thorough analysis of how 64K
base page size kernel stacks up against our current autoMPSS code on the
T2000. If there was a platform where the TLB-vs-.* tradeoffs are obvious,
Niagara would be it.
- Eric
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