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

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Tue, Mar 27, 2018 at 12:23:33AM +0200, Jaromír Dole?ek wrote:

> > Independent of how the scaling problem will be handled, I suggest to
> > increase that window to at least MAXPHYS.
> 
> I think the file mapping window doesn't need to really be tied to MAXPHYS.
> But values over MAXPHYS will probably currently [*] not give further
> significant speedup for filesystem I/O, unless data is being read from cache.

Yes, that's why MAXPHYS might be a good value.

> Did the experiment work?

UBC_WINSHIFT=17 (i.e. 2*MAXPHYS) works fine, but sure, for KVA limited
platforms you have to be careful. On the other hand, it's a temporary
mapping that only exists while doing I/O. How many concurrent I/O
operations would you expect on a 32bit system?


> [*] I hope to get tls-maxphys integrated for 9.0, by then maybe we need
> to revisit this. It would be also nice to revisit the need to map the
> memory to kernel pmap in order to do the I/O.

I'd also like to get another tuning to 8.0. Bumping RA_WINWIZE_MAX
from 8*MAXPHYS to 16*MAXPHYS helps iSCSI and maybe some nvme devices
too.


Greetings,
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."