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Prevent LFC overflow by skipping write when not unpinned pages are available

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This commit is contained in:
Konstantin Knizhnik
2025-01-20 18:19:06 +02:00
parent 02fc58b878
commit d40272f783
1 changed files with 17 additions and 19 deletions
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36
pgxn/neon/file_cache.c
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@@ -913,27 +913,25 @@ lfc_writev(NRelFileInfo rinfo, ForkNumber forkNum, BlockNumber blkno,
}
else
{
/*
* We have two choices if all cache pages are pinned (i.e. used in IO
* operations):
*
* 1) Wait until some of this operation is completed and pages is
* unpinned.
*
* 2) Allocate one more chunk, so that specified cache size is more
* recommendation than hard limit.
*
* As far as probability of such event (that all pages are pinned) is
* considered to be very very small: there are should be very large
* number of concurrent IO operations and them are limited by
* max_connections, we prefer not to complicate code and use second
* approach.
*/
if (lfc_ctl->used >= lfc_ctl->limit && !dlist_is_empty(&lfc_ctl->lru))
if (lfc_ctl->used >= lfc_ctl->limit)
{
/* Cache overflow: evict least recently used chunk */
FileCacheEntry *victim = dlist_container(FileCacheEntry, list_node, dlist_pop_head_node(&lfc_ctl->lru));
FileCacheEntry *victim;
if (dlist_is_empty(&lfc_ctl->lru))
{
/*
* No unpinned entries are available: it should very rarely happen in real life
* because maximal number of backends should be smaller than LFC size.
* So just refuse to update LFC in this case.
*/
neon_log(LOG, "LFC overflow: no unpinned entries available");
hash_search_with_hash_value(lfc_hash, &tag, hash, HASH_REMOVE, NULL);
LWLockRelease(lfc_lock);
return;
}
victim = dlist_container(FileCacheEntry, list_node, dlist_pop_head_node(&lfc_ctl->lru));
for (int i = 0; i < BLOCKS_PER_CHUNK; i++)
{
lfc_ctl->used_pages -= (victim->bitmap[i >> 5] >> (i & 31)) & 1;