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Cool new Free softwareCool new Free softwarePosted Dec 20, 2012 15:15 UTC (Thu) by nix (subscriber, #2304)In reply to: Cool new Free software by andresfreund Parent article: Status.net service to phase out, replaced by pump.io
on my workstation from 2009, with other stuff going on, in an absolutely unoptimized configuration, PG does 320k readonly transactions per sec and 140k trivial writing ones.How? With an absolutely unoptimized default configuration, each commit involves a sync, which involves waiting for the disk. There's no way you can do more than a few hundred such transactions per second unless you have a battery-backed RAID array so you don't need to wait for on-the-average half a disk rotation to bring the data under the head (and then possibly another avg half rotation for associated metadata). Doing 320,000 transactions per second with PostgreSQL is hard to imagine. I am obviously missing something. (Again. It's not my week...)
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Cool new Free software Posted Dec 20, 2012 15:52 UTC (Thu) by andresfreund (subscriber, #69562) [Link]
> How? With an absolutely unoptimized default configuration, each commit involves a sync, which involves waiting for the disk.
unoptimized != default. It was just the system wide installation I had lying around, it sure has some adjustments, just not optimized for anything. I don't know from the top of my head which ones exactly, I can look it up though if you want.
> There's no way you can do more than a few hundred such transactions per second unless you have a battery-backed RAID array so you don't need to wait for on-the-average half a disk rotation to bring the data under the head
Yes, I had synchronous_commit turned off (I do that on all my development systems) thats why I wrote that I had a 1/3 second window of data-loss. Thats ok enough in many scenarios.
You btw. can get into the lower thousands on a single spindle these days (since 9.2) with synchronous commit as well, due to the way WAL flushes are batched/grouped across different sessions. Requires quite a bit of concurrency though.
> (and then possibly another avg half rotation for associated metadata)
PG writes the WAL sequentially into pre-allocated files to avoid that...
> Doing 320,000 transactions per second with PostgreSQL is hard to imagine.
Those were the read only ones. 320k writing transactions to logged (i.e crash-safe) tables is probably impossible independent from the hardware right now due to the locking around the WAL.
Sorry, should have given a bit more context, but it really just was 5 minutes of benchmarking without any seriousness to it.
Cool new Free software Posted Dec 20, 2012 23:23 UTC (Thu) by man_ls (subscriber, #15091) [Link] So there is your "completely unoptimized" database: a heavily customized installation by an expert. Oh, and don't use INSERTs like you were told, just COPYs. And a schemaless specific datatype. And hope that performance is still good... At that point you might as well use a data store which at least has been designed with that scenario in mind, and which any idiot (e.g. me) can use to do thousands of writes per second -- out of the box.
Cool new Free software Posted Dec 21, 2012 0:15 UTC (Fri) by andresfreund (subscriber, #69562) [Link]
Wait what? "Heavily customized"? I now checked and I changed 3 performance critical parameters (and loads of logging/debugging ones, but those don't increase performance).
1) synchronous_commit = off. Is it ok to loose the newest (0.2 - 0.6s) transactions in a crash. Older transactions are still guaranteed to be safe. Thats a choice *YOU* have to make, it really depends the data youre storing (and can be toggled per session & transaction). Obviously things are going to be slower if you require synchronous commits.
2) shared_buffers = 512MB. How much memory can be used to buffer reads/writes. Would I have optimized I would have probably set it to 4-6GB.
3) checkpoint_segments = 40. How much disk can we use for the transaction log. Would I have optimized for benchmarking/write heavy it would have been set up to 300.
I don't know how those could be determined automatically. They depend on what the machine is used for.
And I used plain INSERT & SELECT, no COPY.
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