RE: [Xen-ia64-devel] [PATCH] Enable hash vtlb

["Yang, Fred" <fred.yang@xxxxxxxxx>]

Dan,

>I thought the whole point of this patch was to improve
>performance?

As discussed long time before as well as proposed in Xen Summit as an
option, Hash vTLB is mainly to address SMP scalability for lager system
like Itanium.  It may still matches current global VHPT performance in
UP if take out vTLB's key fetaures for SMP.

>And if the patchset (or a subset of it) *doesn't* fix the
>"gcc segmentation fault" issue AND causes a performance
>degradation AND only fixes a theoretical bug,

This sentence sounds like "unless vTLB fixes gcc, it shouldn't be in"
:-)  Somehow this is a fair call to the real purpose of the hash vTLB.
vTLB is not meant to fix bug, it is to provide scalability feature,
period!  
Please note the GCC issue  is not introduced by vTLB , rather it has
been there long ago!  Community effort is definitely needed to nail this
sneaky bug introduced long ago.

>it should be applied now as it changes enough fundamental
>hypervisor code that it is reasonable to expect that it
>may introduce other subtle bugs.  

As the project goes, we should really decide a patch base on if it is
architecturally needed to support Xen/IPF to achieve its best system
performance, not base on if it changes fundermental code or not!  If a
design is needed, a short-term pain in addressing bugs is better than
long-term unaddressed issues.  




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