Re: [PATCHES] [HACKERS] CIC and deadlocks
Pavan Deolasee [EMAIL PROTECTED] writes: [ patch to reduce probability of deadlock of CREATE INDEX CONCURRENTLY with other things ] This patch no longer applies because of the VirtualXid changes. Looking at it again, I'm fairly dissatisfied with it anyway; I really don't like moving the GetTransactionSnapshot calls around like that, because it opens a risk that GetTransactionSnapshot won't get called at all. Since the autovacuum case is already dealt with separately, I'm thinking there is no problem here that we actually need to solve. C.I.C. can never be guaranteed free of deadlock risk, so I don't see a lot of value in making it free of deadlock risk against just CLUSTER and VACUUM FULL. regards, tom lane -- Sent via pgsql-patches mailing list (pgsql-patches@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-patches
Re: [PATCHES] [HACKERS] CIC and deadlocks
This has been saved for the 8.4 release: http://momjian.postgresql.org/cgi-bin/pgpatches_hold --- Pavan Deolasee wrote: On 4/11/07, Tom Lane [EMAIL PROTECTED] wrote: [ itch... ] The problem is with time-extended execution of GetSnapshotData; what happens if the other guy lost the CPU for a good long time while in the middle of GetSnapshotData? He might set his xmin based on info you saw as long gone. You might be correct that it's safe, but the argument would have to hinge on the OldestXmin process being unable to commit because of someone holding shared ProcArrayLock; a point you are definitely not making above. (Study the comments in GetSnapshotData for awhile, also those in xact.c's commit-related code.) My argument was based on what you said above, but I obviously did not state it well :) Anyways, I think its better to be safe and we agree that its not such a bad thing to take exclusive lock on procarray because CIC is not something that happens very often. Attached is a revised patch which takes exclusive lock on the procarray, rest remaining the same. Thanks, Pavan -- EnterpriseDB http://www.enterprisedb.com [ Attachment, skipping... ] ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq -- Bruce Momjian [EMAIL PROTECTED] http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PATCHES] [HACKERS] CIC and deadlocks
On 4/1/07, Tom Lane [EMAIL PROTECTED] wrote: Good point. I'm envisioning a procarray.c function along the lines of bool TransactionHasSnapshot(xid) which returns true if the xid is currently listed in PGPROC and has a nonzero xmin. CIC's cleanup wait loop would check this and ignore the xid if it returns false. Your point means that this function would have to take exclusive not shared lock while scanning the procarray, which is kind of annoying, but it seems not fatal since CIC isn't done all that frequently. When I looked at the code, it occurred to me that possibly we are OK with just taking shared lock on the procarray. That means that some other transaction can concurrently set its serializable snapshot while we are scanning the procarray. But that should not harm us: if we see the snapshot set, we wait for the transaction. A transaction which is setting its serializable snapshot NOW, can not see the tuples that we did not index, isn't it ? A patch based on the discussion is attached. Thanks, Pavan -- EnterpriseDB http://www.enterprisedb.com CIC_deadlock.patch Description: Binary data ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PATCHES] [HACKERS] CIC and deadlocks
Pavan Deolasee [EMAIL PROTECTED] writes: When I looked at the code, it occurred to me that possibly we are OK with just taking shared lock on the procarray. That means that some other transaction can concurrently set its serializable snapshot while we are scanning the procarray. But that should not harm us: if we see the snapshot set, we wait for the transaction. A transaction which is setting its serializable snapshot NOW, can not see the tuples that we did not index, isn't it ? [ itch... ] The problem is with time-extended execution of GetSnapshotData; what happens if the other guy lost the CPU for a good long time while in the middle of GetSnapshotData? He might set his xmin based on info you saw as long gone. You might be correct that it's safe, but the argument would have to hinge on the OldestXmin process being unable to commit because of someone holding shared ProcArrayLock; a point you are definitely not making above. (Study the comments in GetSnapshotData for awhile, also those in xact.c's commit-related code.) I'm about to head to bed and am certainly in no condition to carry the proof through. Have at it ... regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PATCHES] [HACKERS] CIC and deadlocks
On 4/11/07, Tom Lane [EMAIL PROTECTED] wrote: [ itch... ] The problem is with time-extended execution of GetSnapshotData; what happens if the other guy lost the CPU for a good long time while in the middle of GetSnapshotData? He might set his xmin based on info you saw as long gone. You might be correct that it's safe, but the argument would have to hinge on the OldestXmin process being unable to commit because of someone holding shared ProcArrayLock; a point you are definitely not making above. (Study the comments in GetSnapshotData for awhile, also those in xact.c's commit-related code.) My argument was based on what you said above, but I obviously did not state it well :) Anyways, I think its better to be safe and we agree that its not such a bad thing to take exclusive lock on procarray because CIC is not something that happens very often. Attached is a revised patch which takes exclusive lock on the procarray, rest remaining the same. Thanks, Pavan -- EnterpriseDB http://www.enterprisedb.com CIC_deadlock_v2.patch Description: Binary data ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
