Re: [HACKERS] make async slave to wait for lsn to be replayed

On Wed, Apr 3, 2024 at 10:04 PM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
> On Wed, 3 Apr 2024 at 22:18, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>
>> On Wed, Apr 3, 2024 at 7:55 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>> >
>> > On 2024-Apr-03, Alexander Korotkov wrote:
>> >
>> > > Regarding the shmem data structure for LSN waiters. I didn't pick
>> > > LWLock or ConditionVariable, because I needed the ability to wake up
>> > > only those waiters whose LSN is already replayed. In my experience
>> > > waking up a process is way slower than scanning a short flat array.
>> >
>> > I agree, but I think that's unrelated to what I was saying, which is
>> > just the patch I attach here.
>>
>> Oh, sorry for the confusion. I'd re-read your message. Indeed you
>> meant this very clearly!
>>
>> I'm good with the patch. Attached revision contains a bit of a commit message.
>>
>> > > However, I agree that when the number of waiters is very high and flat
>> > > array may become a problem. It seems that the pairing heap is not
>> > > hard to use for shmem structures. The only memory allocation call in
>> > > paritingheap.c is in pairingheap_allocate(). So, it's only needed to
>> > > be able to initialize the pairing heap in-place, and it will be fine
>> > > for shmem.
>> >
>> > Ok.
>> >
>> > With the code as it stands today, everything in WaitLSNState apart from
>> > the pairing heap is accessed without any locking. I think this is at
>> > least partly OK because each backend only accesses its own entry; but it
>> > deserves a comment. Or maybe something more, because WaitLSNSetLatches
>> > does modify the entry for other backends. (Admittedly, this could only
>> > happens for backends that are already sleeping, and it only happens
>> > with the lock acquired, so it's probably okay. But clearly it deserves
>> > a comment.)
>>
>> Please, check 0002 patch attached. I found it easier to move two
>> assignments we previously moved out of lock, into the lock; then claim
>> WaitLSNState.procInfos is also protected by WaitLSNLock.
>
> Could you re-attach 0002. Seems it failed to attach to the previous message.

I actually forgot both!

------
Regards,
Alexander Korotkov