linux-kernel - Re: [PATCH] xfs: use smaller int param in call to xfs_flush

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Date:	Tue, 22 Apr 2008 14:10:51 +1000
From:	David Chinner <dgc@....com>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	David Chinner <dgc@....com>, Adrian Bunk <bunk@...nel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] xfs: use smaller int param in call to xfs_flush_pages

On Mon, Apr 21, 2008 at 10:18:24PM -0500, Eric Sandeen wrote:
> FWIW this one also seems to make no stack difference, at least on x86_64.
> 
> Not complaining; just checking it out. :)
> 
> If you can shink xfs_bmapi, let me know.  :)

FWIW, the path we care about is this path through ->writepage:

(submit_bio)
_xfs_buf_ioapply		32
xfs_buf_iorequest		0
xfs_buf_iostart			0
xfs_buf_read_flags		0
xfs_trans_read_buf		4
xfs_btree_read_bufs		16
xfs_alloc_lookup		56
xfs_alloc_lookup_eq		16
xfs_alloc_fixup_trees		20
xfs_alloc_ag_vextent_near	76
xfs_alloc_ag_vextent		0
xfs_alloc_vextent		48
xfs_bmap_btalloc		164
xfs_bmap_alloc			0
xfs_bmapi			228
xfs_iomap_write_allocate	116
xfs_iomap			20
xfs_map_blocks			16
xfs_page_state_convert		124
xfs_vm_writepage		12
-------------------------------------
checkstack total:		948

Realistically, the onyl thing we can trim anything off is xfs_bmapi,
xfs_bmap_btalloc, xfs_iomap_write_allocate, and xfs_page_state_convert.
It's going to take a lot of work to get any significant change into
those functions given the complexity of them....

FWIW, if we've come through a syscall, the rest of the trace looks
like:

__writepage				0
write_cache_pages			100
generic_writepages			0
xfs_vm_writepages			12
do_writepages				0
__writeback_single_inode		36
sync_sb_inodes				40
writeback_inodes			0
balance_dirty_pages_ratelimited_nr	76
generic_file_buffered_write		96
xfs_write				80
xfs_file_aio_write			12
do_sync_write				140
vfs_write				12
--------------------------------------------
total					604

So the normal case uses 604 bytes prior to entering ->writepage.

It's when we are already using >2k of the stack when we enter
->writepage that we get into trouble....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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