Re: Fixing excessive shootdowns during FFS read (kern/53124) on x86

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Sun, Mar 25, 2018 at 03:19:28PM -0000, Michael van Elst wrote:
> joerg%bec.de@localhost (Joerg Sonnenberger) writes:
> 
> >What about having a passive unmap as fourth option? I.e. when unmapping
> >in the transfer map, just add them to a FIFO. Process the FIFO on each
> >CPU when there is time or the transfer map runs out of space. Context
> >switching for example would be a natural place to do any such
> >invalidation.
> 
> The mapping is so temporary that it is almost only used on a single CPU.
> Basically it's:
> 
> win = ubc_alloc();
> if (!uiomove(win, ...))
> 	memset(win, 0, ...);
> ubc_release(win, ...);
> 
> For this to be even visible on multiple CPUs, the thread would need
> to migrate to another CPU. Locking down the LWP on a single CPU
> might be the cheapest solution.

Yeah, that's what ephemeral mappings where supposed to be for. The other
question is whether we can't just use the direct map for this on amd64
and similar platforms?

Joerg