Re: mmaping on plan9? (was Re: [9fans] venti /plan9port mmapped

["Alyssa M via 9fans" <9fans@9fans.net>]

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On Thursday, February 12, 2026, at 9:08 AM, hiro wrote:
> you're saying you want the software to be more simple, but at the same time you say you want to exploit some techniques that make the hardware use more efficient.
> the first goal is commendable, but for the second you have to first understand how the hardware actually works. i suggest you share the insight that makes you believe mmap is "more efficient" than reading a large file.
I don't think it is, and I don't think I ever said it was more efficient, did I?
I think having the whole file in your address space makes it easier to select the bits of the file you want to read - assuming you don't need to read all or even most of it.
Being that selective probably only makes a significant speed difference when the file is large, anyway.

I've built a couple of simple disk file systems. I thinking of taking the cache code out of one of them and mapping the whole file system image into the address space - to see how much it simplifies the code. I'm not expecting it will be faster.

On Thursday, February 12, 2026, at 12:38 PM, hiro wrote:
> If you poke at it sparsely then reading it into memory is what you should avoid, thus you use seek and normal read instead. that is more simple, efficient, optimal.
I'm sure that's been argued to death over the years. But in a lot of situations I'd agree with you. It's going to be a tradeoff.

My interest is in simpler software - so I'm exploring a way get some of the effect of mmap without having actual mmap - by making it an implementation detail.
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