linux-kernel - Re: [PATCH] X86: Change the default value of nr_irqs from 32 to NR

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Date:	Thu, 21 Aug 2008 00:23:49 +0100
From:	Alex Nixon <alex.nixon@...rix.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X86: Change the default value of nr_irqs from 32 to NR_IRQs

Alex Nixon wrote:
> Yinghai Lu wrote:
>> when !CONFIG_HAVE_SPARSE_IRQ, with dyn_array, could allocate irq_desc
>> and etc as less as possible.
>> when CONFIG_HAVE_SPARESE_IRQ, no actually meaning for nr_irqs.
>>
>> YH
> 
> So I believe the only case this affects is !CONFIG_HAVE_SPARSE_IRQ
> 
> The worry is that with CONFIG_HAVE_DYN_ARRAY we may waste memory by 
> pre-allocating more irq_descs than may be necessary (NR_IRQs vs 32)?
> 
> With !CONFIG_HAVE_DYN_ARRAY however, a static array of size NR_IRQS is 
> allocated instead - so doesn't defaulting nr_irqs back to NR_IRQS just 
> revert to the old behaviour (with the exception of the irq_descs being 
> allocated in pre_alloc_dyn_array instead)?
> 
> - Alex
> 

Sorry to pester you Yinghai, but I'd like to get a patch for this out 
one way or another as Xen is _completely_ unusable with 5 or more VCPUs.

Can you explain more clearly what the problem with the patch is?

I have a different patch set which solves the problem by adding in a pv 
hook for probe_nr_irqs, but it's by far less clean.

Or alternatively, we could revert your patch b2e5f326bb

Cheers,
- Alex
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