As previously said, we have a Virtuoso with brand new engine multithreading. It is now complete and passes its regular test suite. This is the basis for Virtuoso 5.0, to be available as the open source and commercial cuts as before.
As one benchmark, we used the TPC-C test driver that has always been bundled with Virtuoso. We ran 100000 new orders worth of the TPC-C transaction mix first with one client and then with 4 clients, each client going to its own warehouse, so there was not much lock contention. We did this on a 4 core Intel, the working set in RAM. With the old one, 1 client took 1m43 and 4 clients took 3m47. With the new one, one client took 1m30 and 4 clients took 2m37. So, 400000 new orders in 2m37, for 152820 new orders per minute as opposed to 105720 per minute previously. Do not confuse with the official tpmC metric, that one involves a whole bunch of further rules.
TPC-C has activity spread over a few different tables. With tests dealing with fewer tables, improvements in parallelism are far greater.
Aside from better parallelism, we have other features. One of them is a change in the read committed isolation, so that we now return the previous committed state for uncommitted changed rows instead of waiting for the updating transaction to terminate. This is similar to what Oracle does for read committed. Also we now do log checkpoints without having to abort pending write transactions.
When we have faster inserts, we actually see the RDF bulk loader run slower. This is really backwards. The reason is that while one thread parses, other threads insert and if the inserting threads are done they go to wait on a semaphore and this whole business of context switching absolutely kills performance. With slower inserts, the parser keeps ahead so there is less context switching, hence better overall throughput. I still do not get it how the OS can spend between 1.5 and 6 microseconds, several thousand instructions, deciding what to do next when there are only 3-4 eligible threads and all the rest is background which goes with a few dozen slices per second. Solaris is a little better than Linux at this but not dramatically so. Mac OS X is way worse.
As said, we use Oracle 10G2 on the same platform (Linux FC5 64 bit) for sparring. It is really a very good piece of software. We have written the TPC C transactions in SQL/PL. What is surprising is that these procedures run amazingly slowly, even with a single client. Otherwise the Oracle engine is very fast. Well, as I recall, the official TPC C runs with Oracle use an OCI client and no stored procedures. Strange. While Virtuoso for example fills the initial TPC C state a little faster than Oracle, the procedures run 5-10 times slower with Oracle than with Virtuoso, all data in warm cache and a single client. While some parts of Oracle are really well optimized, all basic joins and aggregates etc, we are surprised at how they could have neglected such a central piece as the PL.
Also, we have looked at transaction semantics. Serializable is mostly serializable with Oracle but does not always keep a steady count. Also it does not prevent inserts into a space that has been found empty by a serializable transaction. True, it will not show these inserts to the serializable transaction, so in this it follows the rules. Also, to make a read really repeatable, it seems that the read has to be FOR UPDATE. Otherwise one can not implement a reliable resource transaction, like changing the balance of an account.
Anyway, the Virtuoso engine overhaul is now mostly complete. This is of course an open ended topic but the present batch is nearing completion. We have gone through as many as 3 implementations of hash joins, some things have yet to be finished there. Oracle has very good hash joins. The only way we could match that was to do it all in memory, dropping any persistent storage of the hash. This is of course OK if the hash is not very large and anyway hash joins go sour if the hash does not fit in working set.
As next topics, we have more RDF and the LUBM benchmark to finish. Also we should revisit TPC-D.
Databases are really quite complicated and extensive pieces of software. Much more so than the casual observer might think.
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Author: Orri Erling
Published: 01/10/2007 15:08 GMT
04/17/2008 21:04 GMT
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