Re: [RFC PATCH] fuse: support splice() reading from fuse device

From: Linus Torvalds
Date: Thu May 20 2010 - 13:52:59 EST

On Thu, 20 May 2010, Miklos Szeredi wrote:
> With Jens' pipe growing patch and additional fuse patches it was
> possible to achieve a 20GBytes/s write throghput on my laptop in a
> "null" filesystem (no page cache, data goes to /dev/null).

Btw, I don't think that is a very interesting benchmark.

The reason I say that is that many man years ago I played with doing
zero-copy pipe read/write system calls (no splice, just automatic "follow
the page tables, mark things read-only etc" things). It was considered
sexy to do things like that during the mid-90's - there were all the crazy
ukernel people with Mach etc doing magic things with moving pages around.

It got me a couple of gigabytes per second back then (when memcpy() speeds
were in the tens of megabytes) on benchmarks like lmbench that just wrote
the same buffer over and over again without ever touching the data.

It was totally worthless on _any_ real load. In fact, it made things
worse. I never found a single case where it helped.

So please don't ever benchmark things that don't make sense, and then use
the numbers as any kind of reason to do anything. It's worse than
worthless. It actually adds negative value to show "look ma, no hands" for
things that nobody does. It makes people think it's a good idea, and
optimizes the wrong thing entirely.

Are there actual real loads that get improved? I don't care if it means
that the improvement goes from three orders of magnitude to just a couple
of percent. The "couple of percent on actual loads" is a lot more
important than "many orders of magnitude on a made-up benchmark".

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