Re: [PATCH] long format VHPT

Matt Chapman
2003-10-02T02:10:54Z

On Thu, Oct 02, 2003 at 11:51:35AM +1000, Peter Chubb wrote:
>
> Aurn> Darren Williams wrote:
> >
> > -#define ia64_rid(ctx,addr) (((ctx) << 3) | (addr >> 61))
> > +#ifdef CONFIG_IA64_LONG_FORMAT_VHPT
> > +#define redistribute_rid(rid) (((rid) & ~0xffff) | (((rid) << 8) & 0xff00) | (((rid) >> 8) & 0xff))
> > +#else
> > +#define redistribute_rid(rid) (rid)
> > +#endif
> > +#define ia64_rid(ctx,addr) redistribute_rid(((ctx) << 3) | (addr >> 61))
> >
>
> Arun> Hi Darren, Can you explain what this code is trying to do ?
>
> If I may comment....
>
> The hash function that the VHPT uses works poorly for consecutive
> RIDs. The redistribute_rid() macro flips around the low order bits to
> get something that hashes more evenly -- otherwise in the experiments
> we did, we saw major hash collision problems.

Yep. The hash function on current processors is basically RID ^ VPN so
if you have close together RIDs and similar address space layout, which
is common (e.g. fork()) you get significant collision problems. This was
a hack to space out the RIDs we present to the hardware without messing
with Linux's sequential allocation scheme. Perhaps there is a better
place to do this.

Matt