Re: LogwrtResult contended spinlock

On 04.09.2020 20:13, Andres Freund wrote:
> Hi,
>
> On 2020-09-04 10:05:45 -0700, Andres Freund wrote:
>> On 2020-09-03 14:34:52 -0400, Alvaro Herrera wrote:
>>> Looking at patterns like this
>>>
>>>     if (XLogCtl->LogwrtRqst.Write < EndPos)
>>>         XLogCtl->LogwrtRqst.Write = EndPos;
>>>
>>> It seems possible to implement with
>>>
>>>      do {
>>>          XLogRecPtr    currwrite;
>>>
>>>          currwrite = pg_atomic_read_u64(LogwrtRqst.Write);
>>>     if (currwrite > EndPos)
>>>              break;  // already done by somebody else
>>>          if (pg_atomic_compare_exchange_u64(LogwrtRqst.Write,
>>>                                        currwrite, EndPos))
>>>              break;  // successfully updated
>>>      } while (true);
>>>
>>> This assumes that LogwrtRqst.Write never goes backwards, so it doesn't
>>> seem good material for a general routine.
>>>
>>> This *seems* correct to me, though this is muddy territory to me.  Also,
>>> are there better ways to go about this?
>> Hm, I was thinking that we'd first go for reading it without a spinlock,
>> but continuing to write it as we currently do.
>>
>> But yea, I can't see an issue with what you propose here. I personally
>> find do {} while () weird and avoid it when not explicitly useful, but
>> that's extremely minor, obviously.
> Re general routine: On second thought, it might actually be worth having
> it. Even just for LSNs - there's plenty places where it's useful to
> ensure a variable is at least a certain size.  I think I would be in
> favor of a general helper function.
Do you mean by general helper function something like this?

void
swap_lsn(XLogRecPtr old_value, XLogRecPtr new_value, bool to_largest)
{
   while (true) {
     XLogRecPtr  currwrite;

     currwrite = pg_atomic_read_u64(old_value);

     if (to_largest)
       if (currwrite > new_value)
         break;  /* already done by somebody else */
     else
       if (currwrite < new_value)
         break;  /* already done by somebody else */

     if (pg_atomic_compare_exchange_u64(old_value,
                        currwrite, new_value))
     break;  /* already done by somebody else */
   }
}


which will be called like
swap_lsn(XLogCtl->LogwrtRqst.Write, EndPos, true);

>
> Greetings,
>
> Andres Freund

This CF entry was inactive for a while. Alvaro, are you going to 
continue working on it?

-- 
Anastasia Lubennikova
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