The updating of the bit array if shared needs to atomic as well, probably with
a read and cas.
> On Dec 1, 2019, at 9:19 PM, Liam <networkimp...@gmail.com> wrote:
>
>
> Oh you've allocated a bit array for every value in the test range, then
> checked for gaps in it?
>
>> On Sunday, December 1, 2019 at 2:21:55 PM UTC-8, Michael Jones 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 <networ...@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). 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.
>>>>>>
>>>>>> --
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>>>>>
>>>>>
>>>>> --
>>>>> Michael T. Jones
>>>>> michae...@gmail.com
>>>>
>>>>
>>>> --
>>>> Michael T. Jones
>>>> michae...@gmail.com
>>>
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>>
>>
>> --
>> Michael T. Jones
>> michae...@gmail.com
>
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