linux-kernel - Re: PAGE_CACHE

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Message-ID: <20090331171431.69ad044f@hobbes>
Date:	Tue, 31 Mar 2009 17:14:31 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: Re: PAGE_CACHE_WC strikes again

On Tue, 31 Mar 2009 17:10:47 -0700
Eric Anholt <eric@...olt.net> wrote:

> I just tracked down what was cutting performance 10x on one of my
> systems on a microbenchmark I'd just written:
> 
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -540,7 +540,7 @@ int drm_gem_mmap(struct file *filp, struct
> vm_area_struct *vma)
>  	/* FIXME: use pgprot_writecombine when available */
>  	prot = pgprot_val(vma->vm_page_prot);
>  #ifdef CONFIG_X86
> -	prot |= _PAGE_CACHE_WC;
> +	/*prot |= _PAGE_CACHE_WC;*/
>  #endif
>  	vma->vm_page_prot = __pgprot(prot);
>  
> Turns out that setting PAGE_CACHE_WC disables the WC effect of the
> MTRR on my non-PAT (disabled due to CPU errata) 945GM system, and this
> workaround took GTT-mapped writes from 120MB/s to 1180MB/s.

What the...  There's a pgprot_writecombine now, but it basically does
the same thing.  Why is WC so broken?  Venki is the fix for this
covered in your last patchset?

-- 
Jesse Barnes, Intel Open Source Technology Center
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