On Mon, Sep 19, 2011 at 8:31 AM, Stephen Frost <sfr...@snowman.net> wrote: > * Benjamin LaHaise (b...@kvack.org) wrote: >> For such tables, can't Postgres track the size of the file internally? I'm >> assuming it's keeping file descriptors open on the tables it manages, in >> which case when it writes to a file to extend it, the internally stored size >> could be updated. Not making a syscall at all would scale far better than >> even a modified lseek() will perform. > > We'd have to have it in shared memory and have a lock around it, it > wouldn't be cheap at all.
In theory, we could implement a lock-free cache. But I still think it would be better to see this fixed on the kernel side. If we had some evidence that all of those lseek() calls were a performance problem even when the i_mutex is not seriously contended, then that would be a good argument for doing this in user-space, but I haven't seen any such evidence. On the other hand, the numbers I posted show that when i_mutex IS contended, it can cause a throughput regression of up to 90%. That seems worth fixing. If it turns out that lseek() is too expensive even in the uncontended case or with the i_mutex contention removed (or if the Linux community is unwilling to accept the proposed fix), then we can (and should) look at further optimizing it within PostgreSQL. My guess, though, is that an unlocked lseek will be fast enough that we won't need to worry about installing our own caching infrastructure (or at least, there will be plenty of more significant performance problems to hunt down first). -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers