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Arrange for read-only accesses to SLRU page buffers to take only a shared

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lock, not exclusive, if the desired page is already in memory.  This can
be demonstrated to be a significant win on the pg_subtrans cache when there
is a large window of open transactions.  It should be useful for pg_clog
as well.  I didn't try to make GetMultiXactIdMembers() use the code, as
that would have taken some restructuring, and what with the local cache
for multixact contents it probably wouldn't really make a difference.
Per my recent proposal.
This commit is contained in:
Tom Lane
2005-12-06 18:10:06 +00:00
parent 953208a34c
commit a615acf555
5 changed files with 145 additions and 28 deletions
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134
src/backend/access/transam/slru.c
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@@ -19,6 +19,10 @@
* reading in or writing out a page buffer does not hold the control lock,
* only the per-buffer lock for the buffer it is working on.
*
* "Holding the control lock" means exclusive lock in all cases except for
* SimpleLruReadPage_ReadOnly(); see comments for SlruRecentlyUsed() for
* the implications of that.
*
* When initiating I/O on a buffer, we acquire the per-buffer lock exclusively
* before releasing the control lock. The per-buffer lock is released after
* completing the I/O, re-acquiring the control lock, and updating the shared
@@ -37,7 +41,7 @@
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.31 2005/11/22 18:17:07 momjian Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.32 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -91,15 +95,30 @@ typedef struct SlruFlushData
} SlruFlushData;
/*
* Macro to mark a buffer slot "most recently used".
* Macro to mark a buffer slot "most recently used". Note multiple evaluation
* of arguments!
*
* The reason for the if-test is that there are often many consecutive
* accesses to the same page (particularly the latest page). By suppressing
* useless increments of cur_lru_count, we reduce the probability that old
* pages' counts will "wrap around" and make them appear recently used.
*
* We allow this code to be executed concurrently by multiple processes within
* SimpleLruReadPage_ReadOnly(). As long as int reads and writes are atomic,
* this should not cause any completely-bogus values to enter the computation.
* However, it is possible for either cur_lru_count or individual
* page_lru_count entries to be "reset" to lower values than they should have,
* in case a process is delayed while it executes this macro. With care in
* SlruSelectLRUPage(), this does little harm, and in any case the absolute
* worst possible consequence is a nonoptimal choice of page to evict. The
* gain from allowing concurrent reads of SLRU pages seems worth it.
*/
#define SlruRecentlyUsed(shared, slotno) \
do { \
if ((shared)->page_lru_count[slotno] != 0) { \
int iilru; \
for (iilru = 0; iilru < NUM_SLRU_BUFFERS; iilru++) \
(shared)->page_lru_count[iilru]++; \
(shared)->page_lru_count[slotno] = 0; \
int new_lru_count = (shared)->cur_lru_count; \
if (new_lru_count != (shared)->page_lru_count[slotno]) { \
(shared)->cur_lru_count = ++new_lru_count; \
(shared)->page_lru_count[slotno] = new_lru_count; \
} \
} while (0)
@@ -165,7 +184,7 @@ SimpleLruInit(SlruCtl ctl, const char *name,
shared->page_buffer[slotno] = bufptr;
shared->page_status[slotno] = SLRU_PAGE_EMPTY;
shared->page_dirty[slotno] = false;
shared->page_lru_count[slotno] = 1;
shared->page_lru_count[slotno] = 0;
shared->buffer_locks[slotno] = LWLockAssign();
bufptr += BLCKSZ;
}
@@ -351,6 +370,49 @@ SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid)
}
}
/*
* Find a page in a shared buffer, reading it in if necessary.
* The page number must correspond to an already-initialized page.
* The caller must intend only read-only access to the page.
*
* The passed-in xid is used only for error reporting, and may be
* InvalidTransactionId if no specific xid is associated with the action.
*
* Return value is the shared-buffer slot number now holding the page.
* The buffer's LRU access info is updated.
*
* Control lock must NOT be held at entry, but will be held at exit.
* It is unspecified whether the lock will be shared or exclusive.
*/
int
SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
{
SlruShared shared = ctl->shared;
int slotno;
/* Try to find the page while holding only shared lock */
LWLockAcquire(shared->ControlLock, LW_SHARED);
/* See if page is already in a buffer */
for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
{
if (shared->page_number[slotno] == pageno &&
shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
{
/* See comments for SlruRecentlyUsed macro */
SlruRecentlyUsed(shared, slotno);
return slotno;
}
}
/* No luck, so switch to normal exclusive lock and do regular read */
LWLockRelease(shared->ControlLock);
LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
return SimpleLruReadPage(ctl, pageno, xid);
}
/*
* Write a page from a shared buffer, if necessary.
* Does nothing if the specified slot is not dirty.
@@ -729,8 +791,10 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
for (;;)
{
int slotno;
int bestslot = 0;
unsigned int bestcount = 0;
int cur_count;
int bestslot;
int best_delta;
int best_page_number;
/* See if page already has a buffer assigned */
for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
@@ -742,17 +806,59 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
/*
* If we find any EMPTY slot, just select that one. Else locate the
* least-recently-used slot that isn't the latest page.
* least-recently-used slot to replace.
*
* Normally the page_lru_count values will all be different and so
* there will be a well-defined LRU page. But since we allow
* concurrent execution of SlruRecentlyUsed() within
* SimpleLruReadPage_ReadOnly(), it is possible that multiple pages
* acquire the same lru_count values. In that case we break ties by
* choosing the furthest-back page.
*
* In no case will we select the slot containing latest_page_number
* for replacement, even if it appears least recently used.
*
* Notice that this next line forcibly advances cur_lru_count to a
* value that is certainly beyond any value that will be in the
* page_lru_count array after the loop finishes. This ensures that
* the next execution of SlruRecentlyUsed will mark the page newly
* used, even if it's for a page that has the current counter value.
* That gets us back on the path to having good data when there are
* multiple pages with the same lru_count.
*/
cur_count = (shared->cur_lru_count)++;
best_delta = -1;
bestslot = 0; /* no-op, just keeps compiler quiet */
best_page_number = 0; /* ditto */
for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
{
int this_delta;
int this_page_number;
if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
return slotno;
if (shared->page_lru_count[slotno] > bestcount &&
shared->page_number[slotno] != shared->latest_page_number)
this_delta = cur_count - shared->page_lru_count[slotno];
if (this_delta < 0)
{
/*
* Clean up in case shared updates have caused cur_count
* increments to get "lost". We back off the page counts,
* rather than trying to increase cur_count, to avoid any
* question of infinite loops or failure in the presence of
* wrapped-around counts.
*/
shared->page_lru_count[slotno] = cur_count;
this_delta = 0;
}
this_page_number = shared->page_number[slotno];
if ((this_delta > best_delta ||
(this_delta == best_delta &&
ctl->PagePrecedes(this_page_number, best_page_number))) &&
this_page_number != shared->latest_page_number)
{
bestslot = slotno;
bestcount = shared->page_lru_count[slotno];
best_delta = this_delta;
best_page_number = this_page_number;
}
}