Re: [Linux-ia64] VHPT performance

[Christian Hildner <christian.hildner@hob.de>]

David

Could you please send me your test program to verify this. Since I haven't fixed the storage in
my test prog maybe there are additional page faults. Is it possible to fix malloc() storage from
userspace?

Thanks

Christian

David Mosberger schrieb:

> >>>>> On Fri, 22 Feb 2002 12:35:51 +0100, Christian Hildner <christian.hildner@hob.de> said:
>
>   Christian> I made some measures and found that on Itanium it takes
>   Christian> about ~700 CPU cycles to load a single byte when TLB is
>   Christian> missing and VHPT is enabled vs. ~900 cycles with VHPT
>   Christian> disabled and handling the TLB miss in the IVT.
>
> It's not that simple.  On Itanium, the VHPT will help only if the TLB
> entry can be found in the cache (this is described in the Itanium
> microarch. manual, IIRC).  I don't think anything has been said
> publically yet what McKinley does, so we'll have to wait a bit longer.
>
> The 700 cycle number sounds too high.  For example, I have a little
> test program that shows repeatedly touching ~92 pages takes about 25
> cycles on average and touching more than 128 pages takes about 73
> cycles on average, for a difference of about 48 cycles.
>
>   Christian> Does
>   Christian> anybody know if the VHPT walker is implemented in
>   Christian> hardware (probably not), microcode (that's what I think)
>   Christian> or unimplemented at all?
>
> It's definitely implemented in hardware, though I don't know the
> implementation details.
>
>         --david