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

From: [email-masked] (Eric W. Biederman)
Date: Mon, 04 Jan 2010 11:16:21 -0800

Yinghai Lu <[email-masked]> writes:

> On Mon, Jan 4, 2010 at 8:55 AM, Eric W. Biederman <[email-masked]> wrote:
>> Yinghai Lu <[email-masked]> writes:
>>
>>> first check with NR_VECTORS - FIRST_EXTERNAL_VECTOR - 0x20
>>> aka minus exceptions and system vectors.
>>>
>>> NR_CPUS = 512, and nr_cpu_ids = 128
>>> will have NR_IRQS = 256 + 512 * 64 = 33024
>>>
>>> assume we have 20 intel ixgbe 6 port cards (with sriov and ixgbevf)
>>>       20 * 6 * 64 * 3 = 23040
>>>
>>> first will get:
>>>       128 * (256 - 64) = 24576
>>> then with nr_irqs_gsi will get
>>>       (120 + 8 * 128 + 120 * 256) = 31864
>>>
>>> so 24576 will be used for nr_irqs.
>>>
>>> 24576 * 8 = 196608 bytes will be used for irq_desc_ptrs[]
>>>
>>> before this patch:
>>>     have nr_irqs = 120 + 8 * 128 + 120 * 64 = 8824
>>>       and irq_desc_ptrs[] is 70592
>>>
>>> Signed-off-by: Yinghai Lu <[email-masked]>
>>
>> I am lost.    arch_probe_nr_irqs appears to be total nonsense.
>>
>> We have three concepts.
>> - The number of irq sources we can talk about.  ( nr_irqs)
>> - The number of irqs we can possibly service.   ((NR_VECTORS - 0x30) *nr_cpu_ids)
>> - The number of irqs we actually connected up to cards in the
>>  system that we need to do something with.
>>
>> Why do we need to allocate arrays at all?
>>
>
> irq_desc is allocated dynamically.
>
> but irq_desc_ptrs is pointer array, it need to be allocated after
> nr_irqs is probed.

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.

Eric