Re: [PATCH v2] Add atomic-box-update! function to (ice-9 atomic)

[Andrew Tropin <andrew@trop.in>]

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On 2023-08-22 19:51, Maxime Devos wrote:

> Op 22-08-2023 om 12:59 schreef Andrew Tropin:
>> 
>> * module/ice-9/atomic.scm (atomic-box-update!): New variable.
>> ---
>> Changes since v1. Use single-argument proc to avoid potential perfomance
>> problems cause of call to apply.
>> 
>>   module/ice-9/atomic.scm | 14 +++++++++++++-
>>   1 file changed, 13 insertions(+), 1 deletion(-)
>> 
>> diff --git a/module/ice-9/atomic.scm b/module/ice-9/atomic.scm
>> index 2a8af901d..6bfa2e8ee 100644
>> --- a/module/ice-9/atomic.scm
>> +++ b/module/ice-9/atomic.scm
>> @@ -25,7 +25,8 @@
>>               atomic-box-ref
>>               atomic-box-set!
>>               atomic-box-swap!
>> -            atomic-box-compare-and-swap!))
>> +            atomic-box-compare-and-swap!
>> +            atomic-box-update!))
>>   
>>   (eval-when (expand load eval)
>>     (load-extension (string-append "libguile-" (effective-version))
>> @@ -36,3 +37,14 @@
>>     (add-interesting-primitive! 'atomic-box-set!)
>>     (add-interesting-primitive! 'atomic-box-swap!)
>>     (add-interesting-primitive! 'atomic-box-compare-and-swap!))
>> +
>> +(define (atomic-box-update! box proc)
>> +  "Atomically updates value of BOX to (PROC BOX-VALUE), returns new
>> +value.
>
> * , returns new value -> and returns the new value
>
> While descriptive works, for consistency with other atomics 
> documentation, this imperative procedure needs to be documented in the 
> imperative mood:
>
> Atomically update the value of BOX to (PROC BOX-VALUE) and return the 
> new value.

Went this way, thank you.

>
> Alternatively, you can adjust the other documentation of atomics to the 
> indicative mood and preserve the original docstring (except for the 
> grammar correction mentioned in the beginning).
>
>    PROC may be called multiple times, and thus PROC should be
>> +free of side effects." > +  (let loop ()
>> +    (let* ((old-value (atomic-box-ref box))
>> +           (new-value (proc old-value)))
>> +      (if (eq? old-value (atomic-box-compare-and-swap! box old-value new-value))
>> +          new-value
>> +          (loop)))))
>
> This can be optimised, by using the return value of CAS as the new old 
> value instead of calling atomic-box-ref again:
>
> (let loop ((old-value (atomic-box-ref box)))
>    (let* ((new-value (proc new-value))
>           (new-old-value (atomic-box-compare-and-swap! box old-value 
> new-value)))
>      (if (eq? new-old-value old-value)
>          new-value
>          (loop new-old-value))))

Make sense, updated the implementation.

> Maybe there is some concurrency weirdness that can cause slower 
> iterations to reduce the number of iterations (*); I'm assuming this 
> isn't the case here.  But if it is, in fact, the case here, and the goal 
> is to exploit this effect, I would think it's better to explicitly 
> implement the back-off.
>
> (I haven't benchmarked any of this, I'm purely going by the number of 
> operations.)
>
> (*) E.g., from Wikipedia: 
> https://en.wikipedia.org/w/index.php?title=Compare-and-swap&oldid=1141385700
>
> [...] Instead of immediately retrying after a CAS operation fails, 
> researchers have found that total system performance can be improved in 
> multiprocessor systems—where many threads constantly update some 
> particular shared variable—if threads that see their CAS fail use 
> exponential backoff—in other words, wait a little before retrying the 
> CAS.[4]

Sent v3.

-- 
Best regards,
Andrew Tropin

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