On Sat, Jan 16, 2016 at 10:23 AM, Amit Kapila <amit.kapil...@gmail.com>
wrote:
> >On Fri, Jan 15, 2016 at 11:23 AM, Mithun Cy <mithun...@enterprisedb.com>
> wrote:
>
>> On Mon, Jan 4, 2016 at 2:28 PM, Andres Freund <and...@anarazel.de> wrote:
>>
>> >> I think at the very least the cache should be protected by a separate
>> >> lock, and that lock should be acquired with TryLock. I.e. the cache is
>> >> updated opportunistically. I'd go for an lwlock in the first iteration.
>>
>
> >I also think this observation of yours is right and currently that is
> >okay because we always first check TransactionIdIsCurrentTransactionId().
>
> >+ const uint32 snapshot_cached= 0;
>
I have fixed all of the issues reported by regress test. Also now when
backend try to cache the snapshot we also try to store the self-xid and sub
xid, so other backends can use them.
I also did some read-only perf tests.
Non-Default Settings.
================
scale_factor=300.
./postgres -c shared_buffers=16GB -N 200 -c min_wal_size=15GB -c
max_wal_size=20GB -c checkpoint_timeout=900 -c maintenance_work_mem=1GB -c
checkpoint_completion_target=0.9
./pgbench -c $clients -j $clients -T 300 -M prepared -S postgres
Machine Detail:
cpu : POWER8
cores: 24 (192 with HT).
Clients Base With cached snapshot
1 19653.914409 19926.884664
16 190764.519336 190040.299297
32 339327.881272 354467.445076
48 462632.02493 464767.917813
64 522642.515148 533271.556703
80 515262.813189 513353.962521
But did not see any perf improvement. Will continue testing the same.
--
Thanks and Regards
Mithun C Y
EnterpriseDB: http://www.enterprisedb.com
diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 5cb28cf..a441ca0 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -1561,6 +1561,7 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap,
elog(ERROR, "cache lookup failed for relation %u", OIDOldHeap);
relform = (Form_pg_class) GETSTRUCT(reltup);
+ Assert(TransactionIdIsNormal(frozenXid));
relform->relfrozenxid = frozenXid;
relform->relminmxid = cutoffMulti;
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 97e8962..8db028f 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -372,11 +372,19 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
(arrayP->numProcs - index - 1) * sizeof(int));
arrayP->pgprocnos[arrayP->numProcs - 1] = -1; /* for debugging */
arrayP->numProcs--;
+
+ pg_atomic_write_u32(&ProcGlobal->snapshot_cached, 0);
+
+ ProcGlobal->cached_snapshot_valid = false;
LWLockRelease(ProcArrayLock);
return;
}
}
+ pg_atomic_write_u32(&ProcGlobal->snapshot_cached, 0);
+
+ ProcGlobal->cached_snapshot_valid = false;
+
/* Ooops */
LWLockRelease(ProcArrayLock);
@@ -420,6 +428,8 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
if (LWLockConditionalAcquire(ProcArrayLock, LW_EXCLUSIVE))
{
ProcArrayEndTransactionInternal(proc, pgxact, latestXid);
+ pg_atomic_write_u32(&ProcGlobal->snapshot_cached, 0);
+ ProcGlobal->cached_snapshot_valid = false;
LWLockRelease(ProcArrayLock);
}
else
@@ -568,6 +578,9 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
nextidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
}
+ pg_atomic_write_u32(&ProcGlobal->snapshot_cached, 0);
+ ProcGlobal->cached_snapshot_valid = false;
+
/* We're done with the lock now. */
LWLockRelease(ProcArrayLock);
@@ -1553,6 +1566,8 @@ GetSnapshotData(Snapshot snapshot)
errmsg("out of memory")));
}
+ snapshot->takenDuringRecovery = RecoveryInProgress();
+
/*
* It is sufficient to get shared lock on ProcArrayLock, even if we are
* going to set MyPgXact->xmin.
@@ -1567,12 +1582,39 @@ GetSnapshotData(Snapshot snapshot)
/* initialize xmin calculation with xmax */
globalxmin = xmin = xmax;
- snapshot->takenDuringRecovery = RecoveryInProgress();
+ if (!snapshot->takenDuringRecovery && ProcGlobal->cached_snapshot_valid)
+ {
+ Snapshot csnap = &ProcGlobal->cached_snapshot;
+ TransactionId *saved_xip;
+ TransactionId *saved_subxip;
- if (!snapshot->takenDuringRecovery)
+ saved_xip = snapshot->xip;
+ saved_subxip = snapshot->subxip;
+
+ memcpy(snapshot, csnap, sizeof(SnapshotData));
+
+ snapshot->xip = saved_xip;
+ snapshot->subxip = saved_subxip;
+
+ memcpy(snapshot->xip, csnap->xip,
+ sizeof(TransactionId) * csnap->xcnt);
+ memcpy(snapshot->subxip, csnap->subxip,
+ sizeof(TransactionId) * csnap->subxcnt);
+ globalxmin = ProcGlobal->cached_snapshot_globalxmin;
+ xmin = csnap->xmin;
+
+ count = csnap->xcnt;
+ subcount = csnap->subxcnt;
+ suboverflowed = csnap->suboverflowed;
+
+ Assert(TransactionIdIsValid(globalxmin));
+ Assert(TransactionIdIsValid(xmin));
+ }
+ else if (!snapshot->takenDuringRecovery)
{
int *pgprocnos = arrayP->pgprocnos;
int numProcs;
+ uint32 snapshot_cached= 0;
/*
* Spin over procArray checking xid, xmin, and subxids. The goal is
@@ -1587,14 +1629,11 @@ GetSnapshotData(Snapshot snapshot)
TransactionId xid;
/*
- * Backend is doing logical decoding which manages xmin
- * separately, check below.
+ * Ignore procs running LAZY VACUUM (which don't need to retain
+ * rows) and backends doing logical decoding (which manages xmin
+ * separately, check below).
*/
- if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
- continue;
-
- /* Ignore procs running LAZY VACUUM */
- if (pgxact->vacuumFlags & PROC_IN_VACUUM)
+ if (pgxact->vacuumFlags & (PROC_IN_LOGICAL_DECODING | PROC_IN_VACUUM))
continue;
/* Update globalxmin to be the smallest valid xmin */
@@ -1663,6 +1702,62 @@ GetSnapshotData(Snapshot snapshot)
}
}
}
+
+ /*
+ * Let only one of the caller cache the computed snapshot, and others
+ * can continue as before.
+ */
+ if (pg_atomic_compare_exchange_u32(&ProcGlobal->snapshot_cached,
+ &snapshot_cached, 1))
+ {
+ Snapshot csnap = &ProcGlobal->cached_snapshot;
+ TransactionId *saved_xip;
+ TransactionId *saved_subxip;
+ int curr_subcount= subcount;
+
+ ProcGlobal->cached_snapshot_globalxmin = globalxmin;
+
+ saved_xip = csnap->xip;
+ saved_subxip = csnap->subxip;
+ memcpy(csnap, snapshot, sizeof(SnapshotData));
+ csnap->xip = saved_xip;
+ csnap->subxip = saved_subxip;
+
+ /* not yet stored. Yuck */
+ csnap->xmin = xmin;
+
+ memcpy(csnap->xip, snapshot->xip,
+ sizeof(TransactionId) * count);
+ memcpy(csnap->subxip, snapshot->subxip,
+ sizeof(TransactionId) * subcount);
+
+ /* Add my own XID to snapshot. */
+ csnap->xip[count] = MyPgXact->xid;
+ csnap->xcnt = count + 1;
+
+ if (!suboverflowed)
+ {
+ if (MyPgXact->overflowed)
+ suboverflowed = true;
+ else
+ {
+ int nxids = MyPgXact->nxids;
+
+ if (nxids > 0)
+ {
+ memcpy(csnap->subxip + subcount,
+ (void *) MyProc->subxids.xids,
+ nxids * sizeof(TransactionId));
+ curr_subcount += nxids;
+ }
+ }
+ }
+
+ csnap->subxcnt = curr_subcount;
+ csnap->suboverflowed = suboverflowed;
+
+ ProcGlobal->cached_snapshot_valid = true;
+ }
}
else
{
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 6453b88..b8d0297 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -51,7 +51,7 @@
#include "storage/spin.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"
-
+#include "port/atomics.h"
/* GUC variables */
int DeadlockTimeout = 1000;
@@ -114,6 +114,13 @@ ProcGlobalShmemSize(void)
size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGXACT)));
size = add_size(size, mul_size(max_prepared_xacts, sizeof(PGXACT)));
+ /* for the cached snapshot */
+#define PROCARRAY_MAXPROCS (MaxBackends + max_prepared_xacts)
+ size = add_size(size, mul_size(sizeof(TransactionId), PROCARRAY_MAXPROCS));
+#define TOTAL_MAX_CACHED_SUBXIDS \
+ ((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
+ size = add_size(size, mul_size(sizeof(TransactionId), TOTAL_MAX_CACHED_SUBXIDS));
+
return size;
}
@@ -278,6 +285,13 @@ InitProcGlobal(void)
/* Create ProcStructLock spinlock, too */
ProcStructLock = (slock_t *) ShmemAlloc(sizeof(slock_t));
SpinLockInit(ProcStructLock);
+
+ /* cached snapshot */
+ ProcGlobal->cached_snapshot_valid = false;
+ pg_atomic_write_u32(&ProcGlobal->snapshot_cached, 0);
+ ProcGlobal->cached_snapshot.xip = ShmemAlloc(PROCARRAY_MAXPROCS * sizeof(TransactionId));
+ ProcGlobal->cached_snapshot.subxip = ShmemAlloc(TOTAL_MAX_CACHED_SUBXIDS * sizeof(TransactionId));
+ ProcGlobal->cached_snapshot_globalxmin = InvalidTransactionId;
}
/*
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index dbcdd3f..73a424e 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -16,6 +16,7 @@
#include "access/xlogdefs.h"
#include "lib/ilist.h"
+#include "utils/snapshot.h"
#include "storage/latch.h"
#include "storage/lock.h"
#include "storage/pg_sema.h"
@@ -234,6 +235,12 @@ typedef struct PROC_HDR
int startupProcPid;
/* Buffer id of the buffer that Startup process waits for pin on, or -1 */
int startupBufferPinWaitBufId;
+
+ /* Cached snapshot */
+ volatile bool cached_snapshot_valid;
+ pg_atomic_uint32 snapshot_cached;
+ SnapshotData cached_snapshot;
+ TransactionId cached_snapshot_globalxmin;
} PROC_HDR;
extern PROC_HDR *ProcGlobal;
--
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