linux-kernel - Re: Downsides to madvise/fadvise(willneed) for application startup

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Date:	Sun, 11 Apr 2010 21:46:02 -0700
From:	Taras Glek <tglek@...illa.com>
To:	drepper@...il.com
CC:	Wu Fengguang <fengguang.wu@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Downsides to madvise/fadvise(willneed) for application startup

On 04/11/2010 09:43 PM, drepper@...il.com wrote:
> On Sun, Apr 11, 2010 at 19:27, Wu Fengguang <fengguang.wu@...el.com> 
> wrote:
>> Yes, every binary/library starts with this 512b read.  It is requested
>> by ld.so/ld-linux.so, and will trigger a 4-page readahead. This is not
>> good readahead. I wonder if ld.so can switch to mmap read for the
>> first read, in order to trigger a larger 128kb readahead.
>
> We first need to know the sizes of the segments and their location in 
> the binary.  The binaries we use now are somewhat well laid out.  The 
> read-only segment starts at offset 0 etc.  But this doesn't have to be 
> the case.  The dynamic linker has to be generic.  Also, even if we 
> start mapping at offset zero, now much to map?  The file might contain 
> debug info which must not be mapped.  Therefore the first read loads 
> enough of the headers to make all of the decisions.  Yes, we could do 
> a mmap of one page instead of the read.  But that's more expansive in 
> general, isn't it?
Can this not be cached for prelinked files? I think it is reasonable to 
optimize the gnu dynamic linker to optimize for an optimal layout 
produced by gnu tools of the same generation.

Taras
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