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
>>>>>
>>>> --
>>>> You received this message because you are subscribed to the Google
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>>>> an email to golan...@googlegroups.com.
>>>> To view this discussion on the web visit
>>>> https://groups.google.com/d/msgid/golang-nuts/4f62dfff-6895-4aaa-9f0d-b635d5ba7ea7%40googlegroups.com
>>>> <https://groups.google.com/d/msgid/golang-nuts/4f62dfff-6895-4aaa-9f0d-b635d5ba7ea7%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>> .
>>>>
>>>> --
>>>> You received this message because you are subscribed to the Google
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>>>> To view this discussion on the web visit
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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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