From: Evgeniy Polyakov <[email protected]>
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.
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