On Mon, Mar 09, 2015 at 09:02:19PM +0000, Mel Gorman wrote:

On Sun, Mar 08, 2015 at 08:40:25PM +0000, Mel Gorman wrote:

quoted

quoted

Because if the answer is 'yes', then we can safely say: 'we regressed 
performance because correctness [not dropping dirty bits] comes before 
performance'.

If the answer is 'no', then we still have a mystery (and a regression) 
to track down.

As a second hack (not to be applied), could we change:

 #define _PAGE_BIT_PROTNONE      _PAGE_BIT_GLOBAL

to:

 #define _PAGE_BIT_PROTNONE      (_PAGE_BIT_GLOBAL+1)

In itself, that's not enough. The SWP_OFFSET_SHIFT would also need updating
as a partial revert of 21d9ee3eda7792c45880b2f11bff8e95c9a061fb but it
can be done.

More importantily, _PAGE_BIT_GLOBAL+1 == the special PTE bit so just
updating the value should crash. For the purposes of testing the idea, I
thought the straight-forward option was to break soft dirty page tracking
and steal their bit for testing (patch below). Took most of the day to
get access to the test machine so tests are not long running and only
the autonuma one has completed;

And the xfsrepair workload also does not show any benefit from using a
different bit either

                                       3.19.0             4.0.0-rc1             4.0.0-rc1             4.0.0-rc1
                                      vanilla               vanilla         slowscan-v2r7        protnone-v3r17
Min      real-fsmark        1164.44 (  0.00%)     1157.41 (  0.60%)     1150.38 (  1.21%)     1173.22 ( -0.75%)
Min      syst-fsmark        4016.12 (  0.00%)     3998.06 (  0.45%)     3988.42 (  0.69%)     4037.90 ( -0.54%)
Min      real-xfsrepair      442.64 (  0.00%)      497.64 (-12.43%)      456.87 ( -3.21%)      489.60 (-10.61%)
Min      syst-xfsrepair      194.97 (  0.00%)      500.61 (-156.76%)      263.41 (-35.10%)      544.56 (-179.30%)
Amean    real-fsmark        1166.28 (  0.00%)     1166.63 ( -0.03%)     1155.97 (  0.88%)     1183.19 ( -1.45%)
Amean    syst-fsmark        4025.87 (  0.00%)     4020.94 (  0.12%)     4004.19 (  0.54%)     4061.64 ( -0.89%)
Amean    real-xfsrepair      447.66 (  0.00%)      507.85 (-13.45%)      459.58 ( -2.66%)      498.71 (-11.40%)
Amean    syst-xfsrepair      202.93 (  0.00%)      519.88 (-156.19%)      281.63 (-38.78%)      569.21 (-180.50%)
Stddev   real-fsmark           1.44 (  0.00%)        6.55 (-354.10%)        3.97 (-175.65%)        9.20 (-537.90%)
Stddev   syst-fsmark           9.76 (  0.00%)       16.22 (-66.27%)       15.09 (-54.69%)       17.47 (-79.13%)
Stddev   real-xfsrepair        5.57 (  0.00%)       11.17 (-100.68%)        3.41 ( 38.66%)        6.77 (-21.63%)
Stddev   syst-xfsrepair        5.69 (  0.00%)       13.98 (-145.78%)       19.94 (-250.49%)       20.03 (-252.05%)
CoeffVar real-fsmark           0.12 (  0.00%)        0.56 (-353.96%)        0.34 (-178.11%)        0.78 (-528.79%)
CoeffVar syst-fsmark           0.24 (  0.00%)        0.40 (-66.48%)        0.38 (-55.53%)        0.43 (-77.55%)
CoeffVar real-xfsrepair        1.24 (  0.00%)        2.20 (-76.89%)        0.74 ( 40.25%)        1.36 ( -9.17%)
CoeffVar syst-xfsrepair        2.80 (  0.00%)        2.69 (  4.06%)        7.08 (-152.54%)        3.52 (-25.51%)
Max      real-fsmark        1167.96 (  0.00%)     1171.98 ( -0.34%)     1159.25 (  0.75%)     1195.41 ( -2.35%)
Max      syst-fsmark        4039.20 (  0.00%)     4033.84 (  0.13%)     4024.53 (  0.36%)     4079.45 ( -1.00%)
Max      real-xfsrepair      455.42 (  0.00%)      523.40 (-14.93%)      464.40 ( -1.97%)      505.82 (-11.07%)
Max      syst-xfsrepair      207.94 (  0.00%)      533.37 (-156.50%)      309.38 (-48.78%)      593.62 (-185.48%)