Shared locking in slru.c - Mailing list pgsql-hackers
From | Tom Lane |
---|---|
Subject | Shared locking in slru.c |
Date | |
Msg-id | 29931.1133376793@sss.pgh.pa.us Whole thread Raw |
Responses |
Re: Shared locking in slru.c
(Kenneth Marshall <ktm@it.is.rice.edu>)
Re: Shared locking in slru.c (Manfred Koizar <mkoi-pg@aon.at>) Re: Shared locking in slru.c (Tom Lane <tgl@sss.pgh.pa.us>) |
List | pgsql-hackers |
I've been fooling around with a test case Rob Creager sent me, which is able to drive PG into a context-switch storm caused by contention for the SubtransControlLock. Rob asked for the test case not to be posted publicly (it's part of some proprietary code), but I found that you can cause some of the same behavior just by running pgbench while holding an open transaction in a psql session. pgbench isn't able to saturate the machine but I think that's due to unrelated pgbench limitations. The basic cause of the problem is that by holding an open transaction while many other transactions come and go, the window of transactions for which pg_subtrans has to be consulted gets wider and wider. If the system is doing a lot of updates, the fraction of tuples for which HeapTupleSatisfiesSnapshot has to invoke SubTransGetTopmostTransaction gets larger too, and so the extent of contention for SubtransControlLock increases, even though no subtransactions are actually in use. I've been looking at various ways to resolve this, but one thing that seems promising is to hack slru.c to take the control lock in shared mode, not exclusive mode, for read-only accesses to pages that are already in memory. The vast majority of accesses to pg_subtrans in the problem scenario are read-only, and so this reduces the need to block and the consequent CS storm. A quick-hack prototype patch for this is attached. (It changes only SubTransGetParent, but if applied of course TransactionIdGetStatus and so on would get the same treatment.) The idea is that we take the lock in shared mode, look to see if the required page is in memory, and if so just return it; if not, release the lock, reacquire in exclusive mode, proceed with the existing code. The reason that the existing code always takes the lock in exclusive mode, even if there's no intention to change data, is that it also needs to adjust the LRU information to reflect the page access, and the way that we're doing that requires exclusive access. So to make the idea work at all, we need some alternative way of managing recency-of-use info. The way the attached patch attacks this is for the shared-lock access case to simply set the page's LRU counter to zero, without bumping up the LRU counters of the other pages as the normal adjustment would do. This is safe to execute in a shared environment since even if someone else is concurrently touching the same page, they'll also be trying to set its counter to zero, and so there's no possibility of ending up in a bad state. However, this leaves us with the likelihood that multiple pages will have equal LRU counters when it comes time to throw out a page to make room for another. The patch deals with that by selecting the furthest-back page of those with the highest LRU setting. I'm not totally happy with this heuristic, though, and was wondering if anyone had a better idea. Anyone seen a lock-free data structure for LRU or approximately-LRU state tracking? regards, tom lane --- src/backend/access/transam/subtrans.c Tue Nov 29 14:34:58 2005 *************** *** 110,118 **** if (!TransactionIdIsNormal(xid)) return InvalidTransactionId; ! LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE); ! ! slotno = SimpleLruReadPage(SubTransCtl, pageno, xid); ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno]; ptr += entryno; --- 110,116 ---- if (!TransactionIdIsNormal(xid)) return InvalidTransactionId; ! slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid); ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno]; ptr += entryno; *** src/backend/access/transam/slru.c.orig Tue Nov 22 16:06:16 2005 --- src/backend/access/transam/slru.c Tue Nov 29 14:34:51 2005 *************** *** 352,357 **** --- 352,395 ---- } /* + * Find a page in a shared buffer, reading it in if necessary. + * The page number must correspond to an already-initialized 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 be held at entry, and will be held at exit. + */ + int + SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid) + { + SlruShared shared = ctl->shared; + int slotno; + + 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) + { + shared->page_lru_count[slotno] = 0; + return slotno; + } + } + + 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. * *************** *** 729,736 **** for (;;) { int slotno; ! int bestslot = 0; ! unsigned int bestcount = 0; /* See if page already has a buffer assigned */ for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++) --- 767,775 ---- for (;;) { int slotno; ! int bestslot; ! unsigned int bestcount; ! int best_page_number; /* See if page already has a buffer assigned */ for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++) *************** *** 744,758 **** * If we find any EMPTY slot, just select that one. Else locate the * least-recently-used slot that isn't the latest page. */ for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++) { 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) { bestslot = slotno; ! bestcount = shared->page_lru_count[slotno]; } } --- 783,808 ---- * If we find any EMPTY slot, just select that one. Else locate the * least-recently-used slot that isn't the latest page. */ + bestslot = 0; + bestcount = 0; + best_page_number = shared->latest_page_number; for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++) { + unsigned int this_lru_count; + int this_page_number; + if (shared->page_status[slotno] == SLRU_PAGE_EMPTY) return slotno; ! this_lru_count = shared->page_lru_count[slotno]; ! this_page_number = shared->page_number[slotno]; ! if ((this_lru_count > bestcount || ! (this_lru_count == bestcount && ! ctl->PagePrecedes(this_page_number, best_page_number))) && ! this_page_number != shared->latest_page_number) { bestslot = slotno; ! bestcount = this_lru_count; ! best_page_number = this_page_number; } }
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