agree, makes sense. also if you trust the compiler, change to use the mod
function, etc.
On Sun, Dec 1, 2019 at 2:31 PM robert engels <reng...@ix.netcom.com> wrote:
> I’d prefer v / 8 over v >> 3 - provides more context in my opinion. The
> compiler will change to right shift if more efficient anyway.
>
> On Dec 1, 2019, at 4:21 PM, Michael Jones <michael.jo...@gmail.com> wrote:
>
> Oh! That's just a bit per integer in the test range 0..total-1. Since Go
> (and everything else) lacks a bit type, I just type such code
> automatically. Bytes hold 8 bits. Array size must be rounded up, so
>
> a := make([]byte, (total+8-1)/8)
>
> array index for test integer n is n/8, so "n>>3"
>
> bit index for the j-th bit, counting up from 0 for the 1's place is "1<<j"
>
> j is n%8, so "n&(8-1)"
>
> if mask=1<<(n&(8-1)) then one can test if the bit is set with
>
> a[n>>3] & mask != 0
>
> to set it is
>
> a[n>>3] |= mask
>
> the values 3 and 8 here are from 8 bits in a byte and 8 = 2**3. if using
> 64-bit ints they become 6 and 64.
>
> On Sat, Nov 30, 2019 at 7:06 PM Liam <networkimp...@gmail.com> wrote:
>
>> I wrote a less-sophisticated version of your test, then realized I'd
>> misspent my time; all I needed was to change the atomic.Add*() to a
>> mutex-protected counter, and see whether my app still failed; it did.
>>
>> But since you took the trouble, I read your code, and would like to
>> understand your collision detector. Could you explain this bit?
>>
>> for _, v := range a {
>> mask := byte(1 << (v & (8 - 1)))
>> index := v >> 3
>>
>> if tally[index]&mask != 0 { ... }
>> ...
>> }
>>
>> On Saturday, November 30, 2019 at 5:33:50 PM UTC-8, Michael Jones wrote:
>>>
>>> As a follow-up, some more timing:
>>>
>>> *47088064 atomic increments/sec (my original email above for heavy
>>> synchronization conflict incrementing)*
>>>
>>> 142049067 atomic increments/sec when each goroutine has its own atomic
>>> update target. (Not testing global synchronization/mutex, just the
>>> overhead of congested vs not.)
>>>
>>> 426232527 ordinary "x++" increments in the workers.
>>>
>>> General idea to remember:
>>>
>>> Atomic increment is ~3x slower than simple add when uncontested.
>>> Highly contested atomic increment is ~3x closer than uncontested,
>>> therefore ~9x-10x slower than simple add.
>>>
>>> 10x is not insignificant, but is nevertheless remarkable for a reliable
>>> atomic operation. This was once, "back in the day", a remarkably expensive
>>> operation, an a feat of genius to accomplish (Dekker's Algorithm
>>> <https://en.wikipedia.org/wiki/Dekker%27s_algorithm>). That it is now
>>> just a number-of-fingers cycles is fantastic progress!
>>>
>>> On Sat, Nov 30, 2019 at 3:38 PM Michael Jones <michae...@gmail.com>
>>> wrote:
>>>
>>>> Liam,
>>>>
>>>> I just wrote a little stress test program for you. Maybe it will make
>>>> you less stressed. ;-)
>>>> https://play.golang.org/p/5_7Geyczd1V
>>>>
>>>> 4 CPU 2016 MacBook Pro:
>>>>
>>>> *celeste:atom mtj$ go run main.go*
>>>> *32 concurrent workers*
>>>> *128 batches of 1048576 atomic increments, 134217728 total increments*
>>>> *2.850 seconds elapsed, 47088064 atomic increments/sec*
>>>> *0 collisions*
>>>>
>>>>
>>>> 18 CPU 2019 iMacPro:
>>>>
>>>> *plum:atom mtj$ go run main.go*
>>>> *32 concurrent workers*
>>>> *128 batches of 1048576 atomic increments, 134217728 total increments*
>>>> *2.730 seconds elapsed, 49167382 atomic increments/sec*
>>>> *0 collisions*
>>>>
>>>>
>>>> Exhaustive demonstration is no proof, but changing the parameters here
>>>> may increase your comfort.
>>>>
>>>> Michael
>>>>
>>>> On Sat, Nov 30, 2019 at 1:02 PM Robert Engels <ren...@ix.netcom.com>
>>>> wrote:
>>>>
>>>>> If this was broken I think a lot of things would break.
>>>>>
>>>>> On Nov 30, 2019, at 1:56 PM, Liam <networ...@gmail.com> wrote:
>>>>>
>>>>>
>>>>> The stress test for my app fails frequently with what looks like a
>>>>> collision in atomic.AddUint64() results, so I wondered whether I had
>>>>> misunderstood atomic-add.
>>>>>
>>>>> So far I can't reproduce it with a small program, so I've probably
>>>>> misunderstood my app :-)
>>>>>
>>>>> On Friday, November 29, 2019 at 6:41:39 PM UTC-8, Kurtis Rader wrote:
>>>>>>
>>>>>> On Fri, Nov 29, 2019 at 6:21 PM Liam <networ...@gmail.com> wrote:
>>>>>>
>>>>>>> Does atomic.AddInt32(&x, 1) always yield unique values for
>>>>>>> concurrent callers?
>>>>>>>
>>>>>>> I'm guessing not, because (I think) I'm seeing that two callers get
>>>>>>> x+2, neither gets x+1.
>>>>>>>
>>>>>>
>>>>>> That shouldn't happen, AFAICT. Can you share the code where the
>>>>>> incorrect behavior is occurring? Or, preferably, a simple reproducer
>>>>>> program?
>>>>>>
>>>>>>
>>>>>>> Is there a way to generate unique values with pkg atomic, or is a
>>>>>>> mutex required?
>>>>>>>
>>>>>>
>>>>>> Keep in mind that atomic.AddInt32() has the usual two's-complement
>>>>>> overflow semantics. If all you want is a generation counter you really
>>>>>> should be using a uint32 and atomic.AddUint32(). Also, depending on your
>>>>>> preferences and performance considerations you might find it preferable
>>>>>> to
>>>>>> use a channel that holds a single int, or small number of ints, that is
>>>>>> fed
>>>>>> by a producer goroutine and consumed by any context needing a uniq ID.
>>>>>> That
>>>>>> makes it easier to abstract the generation of "unique" ints so that they
>>>>>> satisfy other constraints (e.g., they must be even, odd, prime, etc.).
>>>>>>
>>>>>> --
>>>>>> Kurtis Rader
>>>>>> Caretaker of the exceptional canines Junior and Hank
>>>>>>
>>>>>
>>>>> --
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>>>>> <https://groups.google.com/d/msgid/golang-nuts/4f62dfff-6895-4aaa-9f0d-b635d5ba7ea7%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>>> .
>>>>>
>>>>>
>>>>> --
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>>>>> <https://groups.google.com/d/msgid/golang-nuts/C7B99DEA-D183-44EF-9EDA-0B1841AB9DE5%40ix.netcom.com?utm_medium=email&utm_source=footer>
>>>>> .
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> *Michael T. jonesmichae...@gmail.com*
>>>>
>>>
>>>
>>> --
>>>
>>> *Michael T. jonesmichae...@gmail.com*
>>>
>>
>> --
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>> .
>>
>
>
> --
>
> *Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
>
> --
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> .
>
>
>
--
*Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
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