not necessary as the testing and updating is only done in one place by one
the main goroutine.
On Sun, Dec 1, 2019 at 7:46 PM Robert Engels <reng...@ix.netcom.com> wrote:
> 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
>>>> <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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>>>>>> send 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>
>>>>>> .
>>>>>>
>>>>>> --
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>>>>>> Groups "golang-nuts" group.
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>>>>>> send an email to golan...@googlegroups.com.
>>>>>> 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*
>>>>
>>> --
>>> You received this message because you are subscribed to the Google
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>>> .
>>>
>>
>>
>> --
>>
>> *Michael T. jonesmichae...@gmail.com*
>>
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--
*Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
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