linux-kernel - Re: [PATCH] Try to avoid a pessimistic vmalloc() recursion

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Message-ID: <452A77ED.6070001@yahoo.com.au>
Date:	Tue, 10 Oct 2006 02:25:17 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Try to avoid a pessimistic vmalloc() recursion

Eric Dumazet wrote:
> __vmalloc_area_node() is a litle bit pessimist when allocating space for 
> storing struct page pointers.
> 
> When allocating more than 4 MB on ia32, or 2 MB on x86_64,  
> __vmalloc_area_node() has to allocate more than PAGE_SIZE bytes to store 
> pointers to  page structs. This means that two TLB translations are needed to 
> access data.
> 
> This patch tries a kmalloc() call, then only if this first attempt failed, a 
> vmalloc() is performed. (Later, at vfree() time we chose kfree() or vfree() 
> with a test on flags & VM_VPAGES : no change is needed) 
> 
> Most of the time, the first kmalloc() should be OK, so we reduce TLB usage.

But this is only TLB usage when managing (read: freeing) the vmalloc pages,
isn't it? Not when actually accessing the data.

I'd be inclined to NACK this, unless you can show an improvement somewhere:
it is suboptimal to even _try_ allocating higher order pages.

-- 
SUSE Labs, Novell Inc.
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