diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 3049e01e1cd3063bccdfce26e67402dc6872db3e..a8e3f19119a568858e8107bf53196bffa5cfa2c5 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -403,12 +403,6 @@ SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok,
/* Acquire per-buffer lock (cannot deadlock, see notes at top) */
LWLockAcquire(shared->buffer_locks[slotno], LW_EXCLUSIVE);
- /*
- * Temporarily mark page as recently-used to discourage
- * SlruSelectLRUPage from selecting it again for someone else.
- */
- SlruRecentlyUsed(shared, slotno);
-
/* Release control lock while doing I/O */
LWLockRelease(shared->ControlLock);
@@ -909,9 +903,12 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
{
int slotno;
int cur_count;
- int bestslot;
- int best_delta;
- int best_page_number;
+ int bestvalidslot = 0; /* keep compiler quiet */
+ int best_valid_delta = -1;
+ int best_valid_page_number = 0; /* keep compiler quiet */
+ int bestinvalidslot = 0; /* keep compiler quiet */
+ int best_invalid_delta = -1;
+ int best_invalid_page_number = 0; /* keep compiler quiet */
/* See if page already has a buffer assigned */
for (slotno = 0; slotno < shared->num_slots; slotno++)
@@ -922,8 +919,16 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
}
/*
- * If we find any EMPTY slot, just select that one. Else locate the
- * least-recently-used slot to replace.
+ * If we find any EMPTY slot, just select that one. Else choose a
+ * victim page to replace. We normally take the least recently used
+ * valid page, but we will never take the slot containing
+ * latest_page_number, even if it appears least recently used. We
+ * will select a slot that is already I/O busy only if there is no
+ * other choice: a read-busy slot will not be least recently used once
+ * the read finishes, and waiting for an I/O on a write-busy slot is
+ * inferior to just picking some other slot. Testing shows the slot
+ * we pick instead will often be clean, allowing us to begin a read
+ * at once.
*
* Normally the page_lru_count values will all be different and so
* there will be a well-defined LRU page. But since we allow
@@ -932,9 +937,6 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
* 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
@@ -944,9 +946,6 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
* 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 < shared->num_slots; slotno++)
{
int this_delta;
@@ -968,34 +967,57 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
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)
+ if (this_page_number == shared->latest_page_number)
+ continue;
+ if (shared->page_status[slotno] == SLRU_PAGE_VALID)
+ {
+ if (this_delta > best_valid_delta ||
+ (this_delta == best_valid_delta &&
+ ctl->PagePrecedes(this_page_number,
+ best_valid_page_number)))
+ {
+ bestvalidslot = slotno;
+ best_valid_delta = this_delta;
+ best_valid_page_number = this_page_number;
+ }
+ }
+ else
{
- bestslot = slotno;
- best_delta = this_delta;
- best_page_number = this_page_number;
+ if (this_delta > best_invalid_delta ||
+ (this_delta == best_invalid_delta &&
+ ctl->PagePrecedes(this_page_number,
+ best_invalid_page_number)))
+ {
+ bestinvalidslot = slotno;
+ best_invalid_delta = this_delta;
+ best_invalid_page_number = this_page_number;
+ }
}
}
+ /*
+ * If all pages (except possibly the latest one) are I/O busy, we'll
+ * have to wait for an I/O to complete and then retry. In that unhappy
+ * case, we choose to wait for the I/O on the least recently used slot,
+ * on the assumption that it was likely initiated first of all the I/Os
+ * in progress and may therefore finish first.
+ */
+ if (best_valid_delta < 0)
+ {
+ SimpleLruWaitIO(ctl, bestinvalidslot);
+ continue;
+ }
+
/*
* If the selected page is clean, we're set.
*/
- if (shared->page_status[bestslot] == SLRU_PAGE_VALID &&
- !shared->page_dirty[bestslot])
- return bestslot;
+ if (!shared->page_dirty[bestvalidslot])
+ return bestvalidslot;
/*
- * We need to wait for I/O. Normal case is that it's dirty and we
- * must initiate a write, but it's possible that the page is already
- * write-busy, or in the worst case still read-busy. In those cases
- * we wait for the existing I/O to complete.
+ * Write the page.
*/
- if (shared->page_status[bestslot] == SLRU_PAGE_VALID)
- SlruInternalWritePage(ctl, bestslot, NULL);
- else
- SimpleLruWaitIO(ctl, bestslot);
+ SlruInternalWritePage(ctl, bestvalidslot, NULL);
/*
* Now loop back and try again. This is the easiest way of dealing