Followup: [PATCH -mm] make swapin readahead skip over holes

[Dan Magenheimer <dan.magenheimer@oracle.com>]

Hi Rik --

I saw this patch in 3.4-rc1 (because it caused a minor merge
conflict with frontswap) and wondered about its impact.
Since I had a server still set up from running benchmarks
before LSFMM, I ran my kernel compile -jN workload (with
N varying from 4 to 40) on 1GB of RAM, on 3.4-rc2 both with
and without this patch.

For values of N=24 and N=28, your patch made the workload
run 4-9% percent faster.  For N=16 and N=20, it was 5-10%
slower.  And for N=36 and N=40, it was 30%-40% slower!

Is this expected?  Since the swap "disk" is a partition
on the one active drive, maybe the advantage is lost due
to contention?

Thanks,
Dan

commit removed 67f96aa252e606cdf6c3cf1032952ec207ec0cf0

Workload:
	kernel compile "make -jN" with varying N
	measurements in elapsed seconds
	boot kernel: 3.4-rc2
	Oracle Linux 6 distro with ext4
	fresh reboot for each test run
	all tests run as root in multi-user mode

Hardware:
	Dell Optiplex 790 = ~$500
	Intel Core i5-2400 @ 3.10 GHz, 4coreX2thread, 6M cache
	1GB RAM DDR3 1333Mhz (to force swapping)
	One 7200rpm SATA 6.0Gb/s drive with 8MB cache
	10GB swap partition

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