Re: Speed up Clog Access by increasing CLOG buffers

On Tue, Mar 22, 2016 at 4:22 PM, Andres Freund <andres(at)anarazel(dot)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 will try to do so in next version.

> I'm actually rather unconvinced that it's all that common that all
> subtransactions are on one page. If you have concurrency - otherwise
> there'd be not much point in this patch - they'll usually be heavily
> interleaved, no? You can argue that you don't care about subxacts,
> because they're more often used in less concurrent scenarios, but if
> that's the argument, it should actually be made.
>

Note, that we are doing it only when a transaction has less than equal to
64 sub transactions. Now, I am not denying from the fact that there will
be cases where subtransactions won't fall into different pages, but I think
chances of such transactions to participate in group mode will be less and
this patch is mainly targeting scalability for short transactions.

>
> > * 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.

The previous version (v5 - [1]) has code that way, but that adds few extra
instructions for single client case and I was seeing minor performance
regression for single client case due to which it has been changed as per
current code.

> 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
>

Considering above reply to (a), do you want to see it done as a separate
else if loop in patch?

>
> > +
> > +/*
> > + * When we cannot immediately acquire CLogControlLock in exclusive
mode at
> > + * commit time, add ourselves to a list of processes that need their
XIDs
> > + * status update.
>
> At this point my "ABA Problem" alarm goes off. If it's not an actual
> danger, can you please document close by, why not?
>

Why this won't lead to ABA problem is explained below in comments. Refer

+ /*

+ * 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.

+ */

>
> > 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.
>
>
> > + 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.
>

I think it won't create a problem, but surely it is not good to return as
true, will change in next version of patch.

>
> > + 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.
>

It will not matter because we are waking the queued process only once we
are done with xid status update.

>
> 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.
>

Not sure, if I understood which statement you are referring here (are you
referring to atomic read operation) and how can we save the load operation?

>
> > + /*
> > + * 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?
>

We need to remember the head of list to wake up the processes due to which
I think above loop is required.

>
> > diff --git a/src/backend/access/transam/twophase.c
b/src/backend/access/transam/twophase.c
> > index c4fd9ef..120b9c0 100644
> > --- a/src/backend/access/transam/twophase.c
> > +++ b/src/backend/access/transam/twophase.c
> > @@ -177,7 +177,7 @@ static TwoPhaseStateData *TwoPhaseState;
> > /*
> > * Global transaction entry currently locked by us, if any.
> > */
> > -static GlobalTransaction MyLockedGxact = NULL;
> > +GlobalTransaction MyLockedGxact = NULL;
>
> Hm, I'm doubtful it's worthwhile to expose this, just so we can use an
> inline function, but whatever.
>

I have done considering this as a hot-path to save an additional function
call, but can change if you think so.

>
> > +#include "access/clog.h"
> > #include "access/xlogdefs.h"
> > #include "lib/ilist.h"
> > #include "storage/latch.h"
> > @@ -154,6 +155,17 @@ struct PGPROC
> >
> > uint32 wait_event_info; /* proc's wait
information */
> >
> > + /* Support for group transaction status update. */
> > + bool clogGroupMember; /* true, if member of
clog group */
> > + pg_atomic_uint32 clogGroupNext; /* next clog group member
*/
> > + TransactionId clogGroupMemberXid; /* transaction id of clog
group member */
> > + XidStatus clogGroupMemberXidStatus; /*
transaction status of clog
> > +
* group member */
> > + int clogGroupMemberPage; /* clog page
corresponding to
> > +
* transaction id of clog group member */
> > + XLogRecPtr clogGroupMemberLsn; /* WAL location
of commit record for
> > +
* clog group member */
> > +
>
> Man, we're surely bloating PGPROC at a prodigious rate.
>
>
> That's my first pass over the code itself.
>
>
> Hm. Details aside, what concerns me most is that the whole group
> mechanism, as implemented, only works als long as transactions only span
> a short and regular amount of time.
>

Yes, thats the main case which will be targeted by this patch and I think
there are many such cases in OLTP workloads where there are very short
transactions.

>
> If I understand correctly, without having followed the thread, the
> reason you came up with this batching on a per-page level is to bound
> the amount of effort spent by the leader; and thus bound the latency?
>

This is mainly to save the case where multiple pages are not-in-memory and
leader needs to perform the I/O serially. Refer mail [2] for point raised
by Robert.

> 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.
>

I think it will happen that way, but what do we want to see with that
benchmark? I think the results will be that for such a workload either
there is no benefit or will be very less as compare to short transactions.

[1] -
http://www.postgresql.org/message-id/CAA4eK1KUVPxBcGTdOuKyvf5p1sQ0HeUbSMbTxtQc=P65OxiZog@mail.gmail.com
[2] -
http://www.postgresql.org/message-id/CA+TgmoahCx6XgprR=p5==cF0g9uhSHsJxVdWdUEHN9H2Mv0gkw@mail.gmail.com

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com