Re: [PATCH] vmscan: skip freeing memory from zones with lots free

["KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>]

> example,
>
>  Called zone_watermark_ok(zone, 2, pages_min, 0, 0);
>  pages_min  = 64
>  free pages = 80
>
> case A.
>
>     order    nr_pages
>   --------------------
>      2         5
>      1        10
>      0        30
>
>        -> zone_watermark_ok() return 1
>
> case B.
>
>     order    nr_pages
>   --------------------
>      3        10
>      2         0
>      1         0
>      0         0
>
>        -> zone_watermark_ok() return 0

Doh!
this example is obiously buggy.

I guess Mr. KOSAKI is very silly or Idiot.
I recommend to he get feathery blanket and good sleeping, instead
black black coffee ;-)


...but below mesurement result still true.

> This patch change zone_watermark_ok() logic to prefer large contenious block.
>
>
> Result:
>
>  test machine:
>    CPU: ia64 x 8
>    MEM: 8GB
>
>  benchmark:
>    $ tbench 8  (three times mesurement)
>
>    tbench works between about 600sec.
>    alloc_pages() and zone_watermark_ok() are called about 15,000,000 times.
>
>
>              2.6.28-rc6                        this patch
>
>       throughput    max-latency       throughput       max-latency
>        ---------------------------------------------------------
>        1480.92         20.896          1,490.27        19.606
>        1483.94         19.202          1,482.86        21.082
>        1478.93         22.215          1,490.57        23.493
>
> avg     1,481.26        20.771          1,487.90        21.394
> std         2.06         1.233              3.56         1.602
> min     1,478.93        19.202          1,477.86        19.606
> max     1,483.94        22.215          1,490.57        23.493
>
>
> throughput improve about 5MB/sec. it over measurement wobbly.

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