Re: Fixing excessive shootdowns during FFS read (kern/53124) on x86 - emap, direct map

[Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>]

2018-03-31 13:42 GMT+02:00 Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>:
> 2018-03-25 17:27 GMT+02:00 Joerg Sonnenberger <joerg%bec.de@localhost>:
>> 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?
>
> Right, we could/should use emap. I haven't realized emap is actually already
> implemented. It's currently used for pipe for the loan/"direct" write.
>
> I don't know anything about emap thought. Are there any known issues,
> do you reckon it's ready to be used for general I/O handling?

Okay, so I've hacked to gether a patch to switch uvm_bio.c to ephemeral
mapping:

http://www.netbsd.org/~jdolecek/uvm_bio_emap.diff

Seems to boot, no idea what else it will break.

Looking at the state of usage though, the emap is only used for disabled code
path for sys_pipe and nowhere else. That code had several on-and-off passes
for being enabled in 2009, and no further use since then. Doesn't give too much
confidence.

The only port actually having optimization for emap is x86. Since amd64
is also the only one supporting direct map, we are really at liberty to pick
either one. I'd lean towards direct map, since that doesn't require
adding/removing any mapping in pmap_kernel() at all. From looking on the code,
I gather direct map is quite easy to implement for other archs like
sparc64. I'd say
significantly easier than adding the necessary emap hooks into MD pmaps.

Jaromir