Re: [PATCH 2/4] iommu: add a bitmap based dma address allocator

[Jens Axboe <axboe-b10kYP2dOMg@public.gmane.org>]

On 11/25/2015 09:40 AM, Woodhouse, David wrote:
> On Wed, 2015-11-25 at 15:46 +0100, jroedel-l3A5Bk7waGM@public.gmane.org wrote:
>> On Tue, Nov 24, 2015 at 02:05:12PM -0800, Shaohua Li wrote:
>>> The lib/iommu-common.c uses a bitmap and a lock. This implementation
>>> actually uses a percpu_ida which completely avoids locking. It would be
>>> possible to make lib/iommu-common.c use percpu_ida too if somebody wants
>>> to do it, but I think this shouldn't be a blocker for these patches
>>> giving it has huge performance gain.
>>
>> It doesn't "completely avoids locking", the percpu_ida code uses a lock
>> internally too. Also, what is the memory and device address space
>> overhead per cpu?
>
> A percpu lock doesn't bounce cachelines between CPUs very much, so from
> that point of view it might as well not exist :)

As long as the address space can remain more than ~50% empty, it is 
indeed practically lockless. Are we ever worried about higher 
utilization? If so, in my experience percpu_ida fails miserable near or 
at exhaustion.

-- 
Jens Axboe