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

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Date:	Mon, 12 Apr 2010 12:50:29 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"drepper@...il.com" <drepper@...il.com>
Cc:	Taras Glek <tglek@...illa.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 Mon, Apr 12, 2010 at 12:43:00PM +0800, 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,

I once read the ld code, it's more complex than I expected.

> we could do a mmap of one page instead of the read.  But that's more
> expansive in general, isn't it?

Right. Without considering IO, a simple read(512) is more efficient than
mmap()+read+munmap().

Thanks,
Fengguang
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