Re: Fusion-io ioDrive - Mailing list pgsql-performance

On Mon, Jul 7, 2008 at 9:23 AM, Merlin Moncure <mmoncure@gmail.com> wrote:
> I have a lot of problems with your statements.  First of all, we are
> not really talking about 'RAM' storage...I think your comments would
> be more on point if we were talking about mounting database storage
> directly from the server memory for example.  Sever memory and cpu are
> involved to the extent that the o/s using them for caching and
> filesystem things and inside the device driver.

I'm not sure how those cards work, but my guess is that the CPU will
go 100% busy (with a near-zero I/O wait) on any sizable workload.  In
this case, the current pgbench configuration being used is quite small
and probably won't resemble this.

> Also, your comments seem to indicate that having a slower device leads
> to higher concurrency because it allows the process to yield and do
> other things.  This is IMO simply false.

Argue all you want, but this is a fairly well known (20+ year-old) behavior.

> With faster storage cpu loads will increase but only because the overall
> system throughput increases and cpu/memory 'work' increases in terms
> of overall system activity.

Again, I said that response times (throughput) would improve.  I'd
like to see your argument for explaining how you can handle more
CPU-only operations when 0% of the CPU is free for use.

> Presumably as storage approaches speedsof main system memory
> the algorithms of dealing with it will become simpler (not having to
> go through acrobatics to try and making everything sequential)
> and thus faster.

We'll have to see.

> I also find the remarks of software 'optimizing' for strict hardware
> assumptions (L1+L2) cache a little suspicious.  In some old programs I
> remember keeping a giant C 'union' of critical structures that was
> exactly 8k to fit in the 486 cpu cache.  In modern terms I think that
> type of programming (sans some specialized environments) is usually
> counter-productive...I think PostgreSQL's approach of deferring as
> much work as possible to the o/s is a great approach.

All of the major database vendors still see an immense value in
optimizing their algorithms and memory structures for specific
platforms and CPU caches.  Hence, if they're *paying* money for
very-specialized industry professionals to optimize in this way, I
would hesitate to say there isn't any value in it.   As a fact,
Postgres doesn't have those low-level resources, so for the most part,
I have to agree that they have to rely on the OS.

-Jonah