This is the mail archive of the
systemtap@sourceware.org
mailing list for the systemtap project.
Re: tutorial draft checked in
Hi -
hunt wrote:
> [...]
> We have argued this again and again. I see no reason why you want the
> translator to be more complicated and slower. [...]
You misjudge my intention.
> For the specific case of pmaps I am sure I spent more time arguing about
> it than writing it. The disadvantages of what you want to do are
>
> 1. Reader locks are slow. They don't scale as well as per-cpu spinlocks.
At least this is a quantifiable concern.
> 2. The translator holds the lock during the whole probe vs the runtime
> which holds the lock as short a time as possible.
Among other things, this guarantees ACID-style properties for probe
handlers, and prevents various race conditions.
> 3. Having the translator handle low-level locking eliminates the
> possibility of switching the runtime to a more efficient lockless
> solution later.
By removing locks from the runtime that the translator makes
redundant, we still have a "lockless" solution. If locks can be done
away with entirely, the translator can be taught not to emit them.
It's probably one line of code change.
> > Anyway, if the advantage of having unshared per-cpu locks for the <<<
> > case was large, the translator could adopt the technique just as
> > easily.
>
> Obviously not true.
WHAT can you possibly mean by that? The translator could emit per-cpu
spinlocks for pmaps. Its programmer would not even break a sweat.
> It is already done and works in the runtime pmap implementation.
Yes, but the question is where better to put the locking.
> I ran a few benchmarks to demonstrate pmaps scalability and measure the
> additional overhead from the translator reader-writer locks. [...]
Good.
> I ran threads that were making syscalls as fast as possible.
> Results are Kprobes/sec
> 1 thread 4 threads
> Regular 340 500
> Pmaps 340 940
> Pmaps* 380 1040
>
> Pmaps* is pmaps with the redundant reader-writer locks removed.
How about a result with the redundant spinlocks removed?
> Measured overhead of those locks is approximately 10% of the cpu
> time for this test case.
It sounds a bit high, considering all the other overhead involved.
An oprofile count of SMP type events would be interesting.
- FChE