On Tue, Mar 22, 2016 at 4:22 PM, Andres Freund <and...@anarazel.de> wrote: > > Hi, > > On 2016-03-15 10:47:12 +0530, Amit Kapila wrote: > > @@ -248,12 +256,67 @@ set_status_by_pages(int nsubxids, TransactionId *subxids, > > * Record the final state of transaction entries in the commit log for > > * all entries on a single page. Atomic only on this page. > > * > > + * Group the status update for transactions. This improves the efficiency > > + * of the transaction status update by reducing the number of lock > > + * acquisitions required for it. To achieve the group transaction status > > + * update, we need to populate the transaction status related information > > + * in shared memory and doing it for overflowed sub-transactions would need > > + * a big chunk of shared memory, so we are not doing this optimization for > > + * such cases. This optimization is only applicable if the transaction and > > + * all child sub-transactions belong to same page which we presume to be the > > + * most common case, we might be able to apply this when they are not on same > > + * page, but that needs us to map sub-transactions in proc's XidCache based > > + * on pageno for which each time a group leader needs to set the transaction > > + * status and that can lead to some performance penalty as well because it > > + * needs to be done after acquiring CLogControlLock, so let's leave that > > + * case for now. We don't do this optimization for prepared transactions > > + * as the dummy proc associated with such transactions doesn't have a > > + * semaphore associated with it and the same is required for group status > > + * update. We choose not to create a semaphore for dummy procs for this > > + * purpose as the advantage of using this optimization for prepared transactions > > + * is not clear. > > + * > > I think you should try to break up some of the sentences, one of them > spans 7 lines. >
Okay, I have simplified the sentences in the comment. > > > > * Otherwise API is same as TransactionIdSetTreeStatus() > > */ > > static void > > TransactionIdSetPageStatus(TransactionId xid, int nsubxids, > > TransactionId *subxids, XidStatus status, > > - XLogRecPtr lsn, int pageno) > > + XLogRecPtr lsn, int pageno, > > + bool all_xact_same_page) > > +{ > > + /* > > + * If we can immediately acquire CLogControlLock, we update the status > > + * of our own XID and release the lock. If not, use group XID status > > + * update to improve efficiency and if still not able to update, then > > + * acquire CLogControlLock and update it. > > + */ > > + if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE)) > > + { > > + TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno); > > + LWLockRelease(CLogControlLock); > > + } > > + else if (!all_xact_same_page || > > + nsubxids > PGPROC_MAX_CACHED_SUBXIDS || > > + IsGXactActive() || > > + !TransactionGroupUpdateXidStatus(xid, status, lsn, pageno)) > > + { > > + LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); > > + > > + TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno); > > + > > + LWLockRelease(CLogControlLock); > > + } > > +} > > > > This code is a bit arcane. I think it should be restructured to > a) Directly go for LWLockAcquire if !all_xact_same_page || nsubxids > > PGPROC_MAX_CACHED_SUBXIDS || IsGXactActive(). Going for a conditional > lock acquire first can be rather expensive. > b) I'd rather see an explicit fallback for the > !TransactionGroupUpdateXidStatus case, this way it's too hard to > understand. It's also harder to add probes to detect whether that > Changed. > > > > The first process to add itself to the list will acquire > > + * CLogControlLock in exclusive mode and perform TransactionIdSetPageStatusInternal > > + * on behalf of all group members. This avoids a great deal of contention > > + * around CLogControlLock when many processes are trying to commit at once, > > + * since the lock need not be repeatedly handed off from one committing > > + * process to the next. > > + * > > + * Returns true, if transaction status is updated in clog page, else return > > + * false. > > + */ > > +static bool > > +TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, > > + XLogRecPtr lsn, int pageno) > > +{ > > + volatile PROC_HDR *procglobal = ProcGlobal; > > + PGPROC *proc = MyProc; > > + uint32 nextidx; > > + uint32 wakeidx; > > + int extraWaits = -1; > > + > > + /* We should definitely have an XID whose status needs to be updated. */ > > + Assert(TransactionIdIsValid(xid)); > > + > > + /* > > + * Add ourselves to the list of processes needing a group XID status > > + * update. > > + */ > > + proc->clogGroupMember = true; > > + proc->clogGroupMemberXid = xid; > > + proc->clogGroupMemberXidStatus = status; > > + proc->clogGroupMemberPage = pageno; > > + proc->clogGroupMemberLsn = lsn; > > + while (true) > > + { > > + nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst); > > + > > + /* > > + * Add the proc to list, if the clog page where we need to update the > > + * current transaction status is same as group leader's clog page. > > + * There is a race condition here such that after doing the below > > + * check and before adding this proc's clog update to a group, if the > > + * group leader already finishes the group update for this page and > > + * becomes group leader of another group which updates different clog > > + * page, then it will lead to a situation where a single group can > > + * have different clog page updates. Now the chances of such a race > > + * condition are less and even if it happens, the only downside is > > + * that it could lead to serial access of clog pages from disk if > > + * those pages are not in memory. Tests doesn't indicate any > > + * performance hit due to different clog page updates in same group, > > + * however in future, if we want to improve the situation, then we can > > + * detect the non-group leader transactions that tries to update the > > + * different CLOG page after acquiring CLogControlLock and then mark > > + * these transactions such that after waking they need to perform CLOG > > + * update via normal path. > > + */ > > Needs a good portion of polishing. > Okay, I have tried to simplify the comment as well. > > > + if (nextidx != INVALID_PGPROCNO && > > + ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage) > > + return false; > > I think we're returning with clogGroupMember = true - that doesn't look > right. > Changed as per suggestion. > > > + pg_atomic_write_u32(&proc->clogGroupNext, nextidx); > > + > > + if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst, > > + &nextidx, > > + (uint32) proc->pgprocno)) > > + break; > > + } > > So this indeed has ABA type problems. And you appear to be arguing above > that that's ok. Need to ponder that for a bit. > > So, we enqueue ourselves as the *head* of the wait list, if there's > other waiters. Seems like it could lead to the first element after the > leader to be delayed longer than the others. > > > FWIW, You can move the nextidx = part of out the loop, > pgatomic_compare_exchange will update the nextidx value from memory; no > need for another load afterwards. > Changed as per suggestion. > > > + /* > > + * If the list was not empty, the leader will update the status of our > > + * XID. It is impossible to have followers without a leader because the > > + * first process that has added itself to the list will always have > > + * nextidx as INVALID_PGPROCNO. > > + */ > > + if (nextidx != INVALID_PGPROCNO) > > + { > > + /* Sleep until the leader updates our XID status. */ > > + for (;;) > > + { > > + /* acts as a read barrier */ > > + PGSemaphoreLock(&proc->sem); > > + if (!proc->clogGroupMember) > > + break; > > + extraWaits++; > > + } > > + > > + Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO); > > + > > + /* Fix semaphore count for any absorbed wakeups */ > > + while (extraWaits-- > 0) > > + PGSemaphoreUnlock(&proc->sem); > > + return true; > > + } > > + > > + /* We are the leader. Acquire the lock on behalf of everyone. */ > > + LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); > > + > > + /* > > + * Now that we've got the lock, clear the list of processes waiting for > > + * group XID status update, saving a pointer to the head of the list. > > + * Trying to pop elements one at a time could lead to an ABA problem. > > + */ > > + while (true) > > + { > > + nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst); > > + if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst, > > + &nextidx, > > + INVALID_PGPROCNO)) > > + break; > > + } > > Hm. It seems like you should should simply use pg_atomic_exchange_u32(), > rather than compare_exchange? > Changed as per suggestion. > > I think it's worthwhile to create a benchmark that does something like > BEGIN;SELECT ... FOR UPDATE; SELECT pg_sleep(random_time); > INSERT;COMMIT; you'd find that if random is a bit larger (say 20-200ms, > completely realistic values for network RTT + application computation), > the success rate of group updates shrinks noticeably. > Will do some tests based on above test and share results. Attached patch contains all the changes suggested by you. Let me know if I have missed anything or you want it differently. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
group_update_clog_v8.patch
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