Re: [perfbook] Analogy of Figure 7.11 Locking “Saw Kerf”

[Akira Yokosawa <akiyks@gmail.com>]

On Fri, 24 Jun 2022 17:30:53 -0700, Paul E. McKenney wrote:
> On Sat, Jun 25, 2022 at 08:57:35AM +0900, Akira Yokosawa wrote:
>> On Fri, 24 Jun 2022 16:33:16 -0700, Paul E. McKenney wrote:
>>> On Sat, Jun 25, 2022 at 08:12:09AM +0900, Akira Yokosawa wrote:
>>>> Hi Paul,
>>>>
>>>> I find the analogy of Figure 7.11 hard to grasp.
>>>>
>>>> Whether a lock is global or per-instance, the cost of locking
>>>> (saw kerf) is observed only when a CPU/thread does the locking
>>>> operation.
>>>>
>>>> In this figure, does each board represent data elements, not a
>>>> CPU/thread?  If this is the case, what does the waste of "saw kerf"
>>>> mean?
>>>>
>>>> What am I missing?
>>>>
>>>> (I hope I am clear enough on what I don't get...)
>>>
>>> It might well be that I am getting too excited about this one.  ;-)
>>>
>>> Maybe I need to drop it.  At the very least, I need to much more clearly
>>> explain it.
>>>
>>> But...
>>>
>>> Each board represents one lock.  The "saw kerf" is the time lost when
>>> releasing that lock and someone else immediately acquiring it.
>>>
>>> Does that help?
>>
>> Well then, why does the left side figure have ten boards?
> 
> Ten locks.  For example, the single board might correspond to a hash
> table guarded by a single global lock.  The ten boards might correspond
> to a hash table with ten buckets, with per-bucket locking.

Ah, I think I understand what you mean.

On the "Global" side, there is a per-resource locking mechanism
implemented under the protection of a global lock, whereas locks
on the "Per-Instance" side are implemented independently with
each other.

Different boards can be acquired in parallel on both sides of the
figure.

Am I on the same page with you now?

> 
> But it is sounding like this analogy might be more confusing than
> enlightening.

An analogy which needs a lot of explanation might not be a good
analogy...

        Thanks, Akira

> 
> 							Thanx, Paul