Re: [PATCH RFC] btrfs: Simplify locking

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Tejun Heo's message of 2011-03-21 14:11:24 -0400:
> Hello,
> 
> On Mon, Mar 21, 2011 at 01:24:37PM -0400, Chris Mason wrote:
> > Very interesting.  Ok, I'll definitely rerun my benchmarks as well.  I
> > used dbench extensively during the initial tuning, but you're forcing
> > the memory low in order to force IO.
> > 
> > This case doesn't really hammer on the locks, it hammers on the
> > transition from spinning to blocking.  We want also want to compare
> > dbench entirely in ram, which will hammer on the spinning portion.
> 
> Here's re-run of DFL and SIMPLE with the memory restriction lifted.
> Memory is 4GiB and disk remains mostly idle with all CPUs running
> full.
> 
>        USER   SYSTEM   SIRQ    CXTSW  THROUGHPUT
> DFL    59898  504517    377  6814245     782.295
> SIMPLE 61090  493441    457  1631688     827.751
>     
> So, about the same picture.

Ok, this impact of this is really interesting.  If we have very short
waits where there is no IO at all, this patch tends to lose.  I ran with
dbench 10 and got about 20% slower tput.

But, if we do any IO at all it wins by at least that much or more.  I
think we should take this patch and just work on getting rid of the
scheduling with the mutex held where possible.

Tejun, could you please send the mutex_tryspin stuff in?  If we can get
a sob for that I can send the whole thing.

-chris