[9fans] advanced core Linux kernel features not in plan9

[9fans] advanced core Linux kernel features not in plan9

[Yoann Padioleau]

Hi,

I was curious to know which core features of the Linux kernel are not implemented
in the plan9 kernel. By core I mean that I know plan9 does not have all the drivers,
filesystems, buses, etc Linux has, but it has many of its core
features (virtual memory, paging, swapping, demand loading, copy on write, etc),
and even more.

For instance I was not able to find any code related to the buffer cache Linux has.
If you open a big file in a plan9 process, then close it, and later you open it again,
will you pay the IO again? Or is it cached somewhere?

Re: [9fans] advanced core Linux kernel features not in plan9

[erik quanstrom]

> I was curious to know which core features of the Linux kernel are not implemented
> in the plan9 kernel. By core I mean that I know plan9 does not have all the drivers,
> filesystems, buses, etc Linux has, but it has many of its core
> features (virtual memory, paging, swapping, demand loading, copy on write, etc),
> and even more.

on a good day with the right kernel you may get paging with plan 9, but never
swapping.  i've turned it off in my kernels, and it's not coming back.  it's hard to
use with multiple page sizes, and as charles notes, devices are either big enough to
not need it, or too small to have anything to page out to.

> For instance I was not able to find any code related to the buffer cache Linux has.
> If you open a big file in a plan9 process, then close it, and later you open it again,
> will you pay the IO again? Or is it cached somewhere?

there is no buffer cache in plan 9.  file servers do cache, but they
are typically not on the same box, so it should be clear why there is no
buffer cache.

- erik

Re: [9fans] advanced core Linux kernel features not in plan9

[cinap_lenrek]

theres a kernel file cache for cached mounts (see the -C option),
tho its broken in labs plan9.

pages of executables are cached.

the disk fileservers implement a buffer cache to avoid going to disk
and do lazy writing out dirty filesystem blocks.

plan9 is a distributed system. the disk fileservers are really
network fileservers. and the local kernel isnt the only
mutator so it cant invalidate the cache without going to
the fileserver.

--
cinap