linux-kernel - Re: [patch] measurements, numbers about CONFIG_OPTIMIZE

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Date:	Fri, 9 Jan 2009 12:29:18 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dirk Hohndel <hohndel@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	jim owens <jowens@...com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>, jh@...e.cz
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact

On Fri, Jan 09, 2009 at 08:35:06PM +0100, Andi Kleen wrote:
> - Also inline everything static that is only called once
> [on the theory that this shrinks code size which is true
> according to my measurements]
> 
> -fno-inline-functions-called once disables this new rule.
> It's very well and clearly defined.

It's also not necessarily what we want.  For example, in fs/direct-io.c,
we have:

static ssize_t
direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode, 
        const struct iovec *iov, loff_t offset, unsigned long nr_segs, 
        unsigned blkbits, get_block_t get_block, dio_iodone_t end_io,
        struct dio *dio)
{
[150 lines]
}

ssize_t
__blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
        struct block_device *bdev, const struct iovec *iov, loff_t offset, 
        unsigned long nr_segs, get_block_t get_block, dio_iodone_t end_io,
        int dio_lock_type)
{
[100 lines]
        retval = direct_io_worker(rw, iocb, inode, iov, offset,
                                nr_segs, blkbits, get_block, end_io, dio);
[10 lines]
}

Now, I'm not going to argue the directIO code is a shining example of
how we want things to look, but we don't really want ten arguments
being marshalled into a function call; we want gcc to inline the
direct_io_worker() and do its best to optimise the whole thing.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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