[tarantool-patches] Re: [PATCH v2 3/5] Enforce applier out of order protection

[Vladimir Davydov]

On Tue, Jan 29, 2019 at 01:30:40PM +0300, Георгий Кириченко wrote:
> On Monday, January 28, 2019 3:09:01 PM MSK Vladimir Davydov wrote:
> > On Tue, Jan 22, 2019 at 01:31:11PM +0300, Georgy Kirichenko wrote:
> > > Do not skip row until the row is not processed by other appliers.
> > 
> > Looks like a fix for
> > 
> >   https://github.com/tarantool/tarantool/issues/3568
> > 
> > Worth adding a test?
> > 
> > > Prerequisite #980
> > > ---
> > > 
> > >  src/box/applier.cc | 35 ++++++++++++++++++-----------------
> > >  1 file changed, 18 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/src/box/applier.cc b/src/box/applier.cc
> > > index 87873e970..148c8ce5a 100644
> > > --- a/src/box/applier.cc
> > > +++ b/src/box/applier.cc
> > > @@ -504,6 +504,22 @@ applier_subscribe(struct applier *applier)
> > > 
> > >  		applier->lag = ev_now(loop()) - row.tm;
> > >  		applier->last_row_time = ev_monotonic_now(loop());
> > > 
> > > +		struct replica *replica = replica_by_id(row.replica_id);
> > > +		struct latch *latch = (replica ? &replica->order_latch :
> > > +				       &replicaset.applier.order_latch);
> > > +		/*
> > > +		 * In a full mesh topology, the same set
> > > +		 * of changes may arrive via two
> > > +		 * concurrently running appliers. Thanks
> > > +		 * to vclock_follow() above, the first row
> > 
> > I don't see any vclock_follow() above. Please fix the comment.
> > 
> > > +		 * in the set will be skipped - but the
> > > +		 * remaining may execute out of order,
> > > +		 * when the following xstream_write()
> > > +		 * yields on WAL. Hence we need a latch to
> > > +		 * strictly order all changes which belong
> > > +		 * to the same server id.
> > > +		 */
> > > +		latch_lock(latch);
> > > 
> > >  		if (vclock_get(&replicaset.applier.vclock,
> > >  		
> > >  			       row.replica_id) < row.lsn) {
> > >  			
> > >  			if (row.replica_id == instance_id &&
> > 
> > AFAIU this patch makes replicaset.applier.vclock, introduced by the
> > previous patch, useless.
> You are right now, but I plan to release this latch just before commit in case 
> of parallel applier.

Then let's please introduce applier.vclock when we get to implement
parallel applier, because currently I can't say for sure whether we
really need it or not.