linux-kernel - Re: e1000 driver 2.6.18 - how to waste processor cycles

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Date:	Thu, 09 Nov 2006 15:54:08 -0700
From:	"Jeff V. Merkey" <jmerkey@...fmountaingroup.com>
To:	"Jeff V. Merkey" <jmerkey@...eranetworks.com>
CC:	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	NetDEV list <netdev@...r.kernel.org>
Subject: Re: e1000 driver 2.6.18 - how to waste processor cycles

Jeff V. Merkey wrote:

> Jesse Brandeburg wrote:
>
>> On 11/9/06, Jeffrey V. Merkey <jmerkey@...eranetworks.com> wrote:
>>
>>> In the case I am referring to, the memory is already mapped with a
>>> previous call, which means it may be getting
>>> mapped twice.
>>
>>
>>
>> I guess maybe I'm not keeping up with you.  This is what I see looking
>> in 2.6.18, i see e1000_clean_rx_irq:
>
>
>
> Check e1000_alloc_rx_buffers:
>
> if (skb already exists in ring buffer)
>  goto map_skb:
> else
>   dev_alloc_skb
>  ( drop through to map_skb)
>
>
> map_skb:
>  pci_map_single
>
> Jeff


>
>>
>> check done bit
>> pci_unmap_single
>> copybreak and recycle
>> OR
>> hand buffer up stack
>>
>> the only branch before the unmap is the napi break out, and in that
>> case we don't change any memory state, so alloc will not do anything.
>>
>> As for alloc rx, we always map, because we always unmapped.
>
Unmapping every single buffer in rx_irq the remapping them in 
alloc_rx_buffers is wasteful of cycles. 

Jeff

>>
>> Did I miss something?  I would appreciate a more detailed explanation
>> of what you see going wrong.
>>
>>
>
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