Re: [PATCH linux-2.6 01/04] brsem: implement big reader semaphore

[Nick Piggin <nickpiggin@yahoo.com.au>]

Tejun Heo wrote:

>  Other than local_bh_disable/enable(), all brsem read ops do are
> 
>  1. accessing sem->idx
>  2. calculate per-cpu rcnt address from sem->idx
>  3. do one branch on the value of per-cpu rcnt
>  4. inc/dec per-cpu rcnt
> 
>  So, it does access one more cachline and, yeap, there is one branch for 
> bh enabling and several more inside local_bh_enable.  I'll try to get 
> some benchmark numbers for comparison.
> 

Well local_bh_disable touches the preempt count too, although we
can probably assume that's hot in cache.

You might also find yours has a bigger icache footprint as well.

>  I'm thinking about adding down_read(&xxx->s_umount) to write(2) and 
> compare normal rwsem and brsem performance by repeitively writing short 
> data into a file on a UP machine.  Do you have better ideas?
> 

To be honest I'd say that you wouldn't be able to measure it if
you're going through a regular system call path, although such
a measurement certainly won't hurt.

I don't have a better idea though. I don't think a busy loop
microbenchmark is going to be very informative either, it might
actually give a measurable difference but that difference
probably won't be too representitive of real use :\

-- 
SUSE Labs, Novell Inc.

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