[ofa-general] Re: [PATCH] mmu notifiers #v8

[Jack Steiner]

On Mon, Mar 03, 2008 at 08:09:49PM +0200, Avi Kivity wrote:
> Jack Steiner wrote:
> >The range invalidates have a performance advantage for the GRU. TLB 
> >invalidates
> >on the GRU are relatively slow (usec) and interfere somewhat with the 
> >performance
> >of other active GRU instructions. Invalidating a large chunk of addresses 
> >with
> >a single GRU TLBINVAL operation is must faster than issuing a stream of 
> >single
> >page TLBINVALs.
> >
> >I expect this performance advantage will also apply to other users of 
> >mmuops.
> >  
> 
> In theory this would apply to kvm as well (coalesce tlb flush IPIs, 
> lookup shadow page table once), but is it really a fast path?  What 
> triggers range operations for your use cases?
 

Although not frequent, an unmap of a multiple TB object could be quite painful
if each page was invalidated individually instead of 1 invalidate for the entire range.
This is even worse if the application is threaded and the object has been reference by
many GRUs (there are 16 GRU ports per node - each potentially has to be invalidated).

Forks (again, not frequent) would be another case.