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SubjectRe: [PATCH] __cacheline_aligned_in_smp?
In message <> you write:
> From: Rusty Russell <>
> Date: Tue, 14 Jan 2003 12:10:12 +1100
> Hmm, you really want to weakly align it: you don't care if something follo
ws it on
> the cacheline, (ie. don't make it into an array, but it'd be nice if other
> things could share the cacheline) in UP.
> No, that is an incorrect statement.
> I want the rest of the cacheline to be absent of any write-possible
> data. There are many members in there which are read-only and thus
> will only consume a cacheline which would never need to be written
> back to main memory due to modification.

But it's not quite that simple, either. If we say dirty cachelines
cost twice as much as read-only ones (ie. read + write vs. read +
discard), it gives some guide. In particular, if a structure has
struct foo {
readonly R;
writeable W;

And it normally fits in one cacheline, but you set the alignment of W
to a cacheline, now it fits in two, you've lost. (Note, struct
tcp_hashinfo is not such a structure, this is just talking to the

> You really don't understand what I'm trying to accomplish.

No. Thanks for the explanation.

> I want alignment on cache line boundary, and I don't want anything
> else in that cacheline.

A "read-mostly" section might be appropriate, then. Of course, you'd
have to split the structure, in that case, and it's not worth it if
there are only a few of these.

Have I finally got it through my thick skull now?
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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