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

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

On Sat, Mar 24, 2018 at 08:42:34PM +0100, Jaromír Doleček wrote:
> The problem there is that FFS triggers a pathologic case. I/O transfer maps
> and then unmaps each block into kernel pmap, so that the data could be
> copied into user memory. This triggers TLB shootdown IPIs for each FS
> block, sent  to all CPUs which happen to be idle, or otherwise running on
> kernel pmap. On systems with many idle CPUs these TLB shootdowns cause a
> lot of synchronization overhead.

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.

Joerg