On Sun, 13 Oct 2019 at 19:50, Amit Kapila <amit.kapil...@gmail.com> wrote:
> On Sun, Oct 13, 2019 at 12:25 PM Dilip Kumar <dilipbal...@gmail.com> > wrote: > > > > On Thu, Oct 3, 2019 at 2:43 PM Amit Kapila <amit.kapil...@gmail.com> > wrote: > > > > > > 3. > > > + * > > > + * While updating the existing change with detoasted tuple data, we > need to > > > + * update the memory accounting info, because the change size will > differ. > > > + * Otherwise the accounting may get out of sync, triggering > serialization > > > + * at unexpected times. > > > + * > > > + * We simply subtract size of the change before rejiggering the > tuple, and > > > + * then adding the new size. This makes it look like the change was > removed > > > + * and then added back, except it only tweaks the accounting info. > > > + * > > > + * In particular it can't trigger serialization, which would be > pointless > > > + * anyway as it happens during commit processing right before handing > > > + * the change to the output plugin. > > > */ > > > static void > > > ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn, > > > @@ -3023,6 +3281,13 @@ ReorderBufferToastReplace(ReorderBuffer *rb, > ReorderBufferTXN *txn, > > > if (txn->toast_hash == NULL) > > > return; > > > > > > + /* > > > + * We're going modify the size of the change, so to make sure the > > > + * accounting is correct we'll make it look like we're removing the > > > + * change now (with the old size), and then re-add it at the end. > > > + */ > > > + ReorderBufferChangeMemoryUpdate(rb, change, false); > > > > > > It is not very clear why this change is required. Basically, this is > done at commit time after which actually we shouldn't attempt to spill > these changes. This is mentioned in comments as well, but it is not clear > if that is the case, then how and when accounting can create a problem. If > possible, can you explain it with an example? > > > > > IIUC, we are keeping the track of the memory in ReorderBuffer which is > > common across the transactions. So even if this transaction is > > committing and will not spill to dis but we need to keep the memory > > accounting correct for the future changes in other transactions. > > > > You are right. I somehow missed that we need to keep the size > computation in sync even during commit for other in-progress > transactions in the ReorderBuffer. You can ignore this point or maybe > slightly adjust the comment to make it explicit. Does anyone object if we add the reorder buffer total size & in-memory size to struct WalSnd too, so we can report it in pg_stat_replication? I can follow up with a patch to add on top of this one if you think it's reasonable. I'll also take the opportunity to add a number of tracepoints across the walsender and logical decoding, since right now it's very opaque in production systems ... and everyone just LOVES hunting down debug syms and attaching gdb to production DBs. -- Craig Ringer http://www.2ndQuadrant.com/ 2ndQuadrant - PostgreSQL Solutions for the Enterprise
