From mboxrd@z Thu Jan  1 00:00:00 1970
From: "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH 0/8] Intel I/O Acceleration Technology (I/OAT)
Date: Sat, 04 Mar 2006 18:08:19 -0800 (PST)
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From: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Date: Sun, 5 Mar 2006 04:43:25 +0300

> According to investigation made for kevent based FS AIO reading,
> get_user_pages() performange graph looks like sqrt() function
> with plato starting on about 64-80 pages on Xeon 2.4Ghz with 1Gb of ram,
> while memcopy() is linear, so it can be noticebly slower than
> copy_to_user() if get_user_pages() is used aggressively, so userspace
> application must reuse the same, already grabbed buffer for maximum
> performance, but Intel folks did not provide theirs usage case and any
> benchmarks as far as I know.

Of course, and programming the DMA controller has overhead
as well.  This is why would would not use I/O AT with small
transfer sizes.