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SubjectRe: Horrible L2 cache effects from kernel compile
Date
In article <3E5BB7EE.5090301@colorfullife.com>,
Manfred Spraul <manfred@colorfullife.com> wrote:
>
>Are you sure that this will help?

It might, under some loads.

However, I don't think it's a long-term solution, since the hashing will
mean that for any reasonably spread out load you _will_ always walk all
dentries.

So the long-term solution is to either use a local lookup (which we
ended up doing for the page cache) _or_ to limit the number of dentries
themselves some way. The latter sounds like a bad idea.

>Btw, has anyone tried to replaced the global dcache with something
>local, perhaps a tree instead of d_child, and then lookup in d_child_tree?

I'd love to see somebody try. The main worry is the overhead required
per directory dentry and keeping it scalable. The dentry tree _will_ be
quickly populated and one common case is a few huge directories, yet at
the same time for most dentries there won't be any children at all or
very few of them).

Right now the "child" list is just a simple linked list, and changing
that to something more complex might make it possible to get rid of the
hash entirely. But increasing the size of individual dentries is a bad
idea, so it would have to be something fairly smart.

Linus
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