Re: Reducing ClogControlLock contention - Mailing list pgsql-hackers

On Wed, Jul 1, 2015 at 3:49 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
> On 1 July 2015 at 11:14, Andres Freund <andres@anarazel.de> wrote:
>>
>> On 2015-07-01 09:08:11 +0100, Simon Riggs wrote:
>> > On 1 July 2015 at 09:00, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> >
>> > > On Tue, Jun 30, 2015 at 12:32 PM, Simon Riggs <simon@2ndquadrant.com>
>> > > a.  the semantics of new LWLock (CommitLock) introduced
>> > > by patch seems to be different in the sense that it is just taken in
>> > > Exclusive mode (and no Shared mode is required) as per your proposal. We
>> > > could use existing LWLock APi's, but on the other hand we could even
>> > > invent new LWLock API for this kind of locking.
>> > >
>> >
>> > LWLock API code is already too complex, so -1 for more changes there
>>
>> I don't think that's a valid argument. It's better to have the
>> complexity in one place (lwlock) than have rather similar complexity in
>> several other places. The clog control lock is far from the only place
>> that would benefit from tricks along these lines.
>
>
> What "tricks" are being used??
>
> Please explain why taking 2 locks is bad here, yet works fine elsewhere.
>

One thing that could be risky in this new scheme of locking

is that now in functions TransactionIdSetPageStatus and

TransactionIdSetStatusBit, we modify slru's shared state with Control Lock

in Shared mode whereas I think it is mandated in the code that those

should be modified with ControlLock in Exlusive mode.  This could have

some repercussions.

Another thing is that in this flow, with patch there will be three locks

(we take per-buffer locks before doing I/O) that will get involved rather than

two, so one effect of this patch will be that currently while doing I/O,

concurrent committers will be allowed to proceed as we release ControlLock

before doing the same whereas with Patch, they will not be allowed as they

are blocked by CommitLock.  Now may be this scenario is less common and

doesn't matter much if the patch improves the more common scenario,

however this is an indication of what Andres tries to highlight that having more

locks for this might make patch more complicated.