Subject: Re: [PATCH 2/2] x86: get more exact nr_irqs

From: [email-masked] (Eric W. Biederman)
Date: Mon, 04 Jan 2010 12:05:08 -0800

"H. Peter Anvin" <[email-masked]> writes:

> On 01/04/2010 11:16 AM, Eric W. Biederman wrote:
>> 
>> If we care about memory use efficiency let's replace irq_desc_ptrs
>> with a rbtree or a radix_tree.  Something that moves the memory use
>> penalty onto those machines that have a lot of irqs.
>> 
>
> rbtree doesn't make much sense for something that is addressed by index,
> and doesn't need to answer questions of the form "give me the highest
> member <= X".  A hash table or radix tree makes sense, depending on the
> expected sparseness of the index.

Not counting irqs for msi's I think we are looking 36% to 25% fill.  Maybe
a little lower.  The sparseness is much higher if we count the number of
irqs that we might/use allocate as we do today.

Short of driver hotplug msis should be allocated densely, unless we start
reserving all possible 4K msi-x vectors.

For each ioapic we allocate 16 gsis, and only maybe four of them are
connected to actual pci slots.

This is essentially a slow path operation, so as long as we are not
too expensive we can use any data structure we want.  In kernel hash
tables don't grow well so I don't think a hash table is a good choice,
and a hash table is essentially what we have now.

The truth is we don't know how many irqs we will have until msi
supporting drivers claim all of theirs.

I think a radix-tree would likely be the least intrusive choice as it
does not imply any changes to the data structure indexed.

Eric