Linus Torvalds (torvalds@osdl.org) on 5/15/06 wrote:
---------------------------
>Brendan (btrotter@gmail.com) on 5/15/06 wrote:
>>
>>I _do_ understand this, I just don't see how seperating
>>everything so that only one process has access to each
>>"thing" would make scalability suck or make me 10 years
>>behind.
>
>Seperating some things is a wonderful thing, and not ten
>years behind. Almost all lockless algorithms depend on
>per-cpu data structures, and that's fine.

By "10 years behind", I was more referring to Brendan's
assertion that cacheline contention doesn't matter because
it is obscured by lock contention.

>>If anything, a system where the device driver runs in the
>>context of some other process would have worse cacheline
>>contention and worse scalability because of the
>>device driver accessing it's own local data from any CPU
>>that happened to run it.
>
>What local data?
>
>Last I saw, there's basically no local data except for
>the data that needs to be shared anyway (ie, the queue),
>and the data you end up having to work with (which is
>coming from the CPU that happened to run it).

Exactly. You can't pretend that the message queue is zero
cost because of the assertion that everything will be backed
up behind locks anyway.

(they may *appear* to be free to the observer, because of
all the overheads involved in context switching and/or
sending messages to remote CPUs...)