Older blog entries for alvherre (starting at number 2)

phantoms – a better idea

After wasting several days trying to come up with a way to make Postgres' lock manager spill to disk, I gave up and ICQed Bruce. This was mainly motivated by the fact that several people complained that the idea of having any process sleep on I/O waiting for the lock manager was a loser. After thinking on it for a while I cannot but agree. I even considered writing some sort of lightweight B-Tree handler on top of SLRU, so we could quickly find lock entries on disk by their LOCKTAG; this is better than using a plain sequential structure, but it's difficult to get right, it's lots of additional code, and it will still need I/O for getting locks :-(

So when I talked to Bruce, he simply said it was a bad idea to use the lock manager to lock tuples; we can't afford to have that many lock entries. In a way it was a disheartening idea. He wondered that maybe we should use some sort of Phantom TransactionIds, an idea on which hearing I almost freeze on terror – I had already heard of Phantom Xids from him, during the nested transactions project, and it was very painful (to me) to code and in the end it was an utter failure. (Althought coding that was a very interesting exercise from which I learned a thing or three – the patches and the disappointing comments from Tom are here.)

So I spent the next day thinking on phantom Ids and felt miserable ;-) because I didn't really see how would they apply to this case.

Turns out it's surprinsingly easy to have it all fit together. Phantom Xids in this case are just a means of indicating a set of TransactionIds; and we store the set on SLRU. So when somebody wants to lock a row, he creates a set; and when somebody needs to lock exclusively, he can sleep on every TransactionId of the set. The only tricky part is how to store variable length content on SLRU, but that is easily solved by using two SLRU areas.

So far I have coded a good deal of the whole thing; though it still doesn't do everything yet, I have now confirmed that the idea is perfectly workable and I'm sure it will have perfect performance.

This means we will have shared row locks for 8.1! I can't help but feel good about that. No more deadlocks in foreign keys. Cool!

posted the patch

Yeah, apparently what I had first thought was in fact the right idea. I'm talking about my shared-row-locks project. Due to some misunderstanding on my part, I figured that the simpler idea was wrong; so I tried a lot of other things, and of course, they were also wrong. So eventually I came back and tried the first thing again, and discovered that it works as expected (by me at least). So I posted the patch, and promptly received a comment from Tom which made me notice a gross mistake. Easily solved, but gross anyway ;-)

performance measure stupidity

I ran some performance testing to verify that my patch won't make people too angry at me. I was terrified to discover that it had dropped by 25% or so in pgbench. I spent an hour and a half looking at the patch searching for the culprit (I didn't want to compile with profiling enabled because my machine is somewhat slow) ... And then I realized that I had compiled the whole backend/access/heap directory with -O0. Recompiled, reran pgbench and now I see no measurable difference between pristine sources and my tree. That's fortunate at least. I still have to see how will the lock-spilling code perform.

28 Mar 2005 (updated 28 Mar 2005 at 15:04 UTC) »

Finally got around to creating a blog. Hopefully I'll be posting with some regularity here.

I'll begin with my current activities: I've been busy these days trying to make Postgres use shared locks, not exclusive locks, in foreign key checking. This is not as easy as it sounds, because the current locking system is limited in available memory, which means the current row-locking code does not use it at all; instead it marks tuples directly on disk, which in turn means we can't have more than one locker at a time. Hence exclusive locking. Moreover, the heap access method is not at all prepared to deal cleanly with multiple lockers. (I also want to get rid of duplicated code in the heap AM, which confuses me because the duplication is not verbatim and so it needs slight adjustments before refactoring can be done.)

So, I've been mostly reading existing code and dumping one solution after another. But eventually I'll come to it — in fact, I think I found a real solution this time that I expect to be able to try tomorrow.

Additionally, I submitted a patch to allow tracking dependencies on shared objects (users, groups, tablespaces). This means that we will be able to deny dropping a user which owns objects in other databases. IMHO this is not a very exciting new feature for the database, but for me it means correcting annoying and buggy behavior. Tom Lane found the patch missing in several respects; I'll be correcting it as soon as I get rid of the shared-row-locking issue. Hopefully I'll be able to finish both things before 8.1.

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