linux-kernel - Re: [PATCH 4/5] aio: vmap ringbuffer

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Message-ID: <20121009182949.GO26187@lenny.home.zabbo.net>
Date:	Tue, 9 Oct 2012 11:29:49 -0700
From:	Zach Brown <zab@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, tytso@....edu
Subject: Re: [PATCH 4/5] aio: vmap ringbuffer

On Mon, Oct 08, 2012 at 11:39:19PM -0700, Kent Overstreet wrote:
> It simplifies a lot of stuff if the ringbuffer is contiguously mapped
> into kernel space, and we can delete a lot of code - in particular, this
> is useful for converting read_events() to cmpxchg.

1) I'm concerned that Our Favourite Database Benchmarkers will see an
effect of having more TLB pressure from that mapping.  kmap_atomic()
should be cheaper.  We'll need to measure the difference.

2) I'm not at all convinced that the current ring semantics are safe
with cmpxchg(), more in the next patch.

- z
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