Re: major performance drop on raid5 due to context switches caused by small max_hw_sectors [partially resolved]

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On Sun, 22 Apr 2007, Pallai Roland wrote:


On Sunday 22 April 2007 10:47:59 Justin Piszcz wrote:
On Sun, 22 Apr 2007, Pallai Roland wrote:
On Sunday 22 April 2007 02:18:09 Justin Piszcz wrote:

How did you run your read test?


I did run 100 parallel reader process (dd) top of XFS file system, try
this: for i in `seq 1 100`; do dd of=$i if=/dev/zero bs=64k 2>/dev/null;
done for i in `seq 1 100`; do dd if=$i of=/dev/zero bs=64k 2>/dev/null &
done

and don't forget to set max_sectors_kb below chunk size (eg. 64/128Kb)
/sys/block# for i in sd*; do echo 64 >$i/queue/max_sectors_kb; done

I also set 2048/4096 readahead sectors with blockdev --setra

You need 50-100 reader processes for this issue, I think so. My kernel
version is 2.6.20.3


In one xterm:
for i in `seq 1 100`; do dd of=$i if=/dev/zero bs=64k 2>/dev/null; done

In another:
for i in `seq 1 100`; do dd if=/dev/md3 of=$i.out bs=64k & done

Write and read files top of XFS, not on the block device. $i isn't a typo, you
should write into 100 files and read back by 100 threads in parallel when
done. I've 1Gb of RAM, maybe you should use mem= kernel parameter on boot.

1. for i in `seq 1 100`; do dd of=$i if=/dev/zero bs=1M count=100 2>/dev/null;
done
2. for i in `seq 1 100`; do dd if=$i of=/dev/zero bs=64k 2>/dev/null & done


--
d


I use a combination of 4 Silicon Image controllers (SiI) and the Intel 965 chipset. My max_sectors_kb is 128kb, my chunk size is 128kb, why do you set the max_sectors_kb less than the chunk size? For read-ahead, there are some good benchmarks by SGI(?) I believe and some others that states 16MB is the best value, over that, you lose on reads/writes or the other, 16MB appears to be optimal for best overall value. Do these values look good to you, or?

Read 100 files on XFS simultaneously:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0    104 146680    460 3179400    0    0   138    17   77  161  1  0 99  0
 0  0    104 146680    460 3179400    0    0     0     0 1095  190  0  0 100  0
16 84    104 149024    460 3162880    0    0 57492    25 1481 1284  2 10 88  1
 0 88    104 148728    396 2982628    0    0 296904     8 2816 4728  1 48  0 51
 0 88    104 147796     40 2894752    0    0 204080     0 2815 3507  0  8  0 92
 0 87    104 149272     40 2845688    0    0 116200     0 2781 2836  0  6  0 94
 0 87    104 147872     40 2770888    0    0 188260     0 2948 3043  0  9  0 91
 0 87    104 148244     40 2583700    0    0 218440     0 2856 3218  0 12  0 88
 1 87    104 147528     40 2385668    0    0 181352     0 3225 4211  1  9  0 90
 0 87    104 147784     40 2343868    0    0 225716     0 3205 3725  0  9  0 91
 0 88    104 146112     40 2233272    0    0 202968     8 3956 5669  0  9  0 91
 0 88    104 146452     40 2325332    0    0 241196     0 4374 4927  0  7  0 93
 0 88    104 147792     40 2463600    0    0 239788     0 3216 1139  1  5  0 94
 1 88    104 148600     40 2468352    0    0 187488     0 3107 2883  0  6  0 94
 0 88    104 147240     40 2553940    0    0 168236     0 3559 2795  0  5  0 95
 0 87    104 147772     40 2682932    0    0 278092     0 5531 7507  0  9  0 91
 0 87    104 148200     40 2813376    0    0 201324     0 4346 5726  0  6  0 94
 1 87    104 148196     40 2774932    0    0 193620     0 5433 11595  0 10  0 90
 0 87    104 148488     40 2785256    0    0 227568     0 4081 6121  0 11  0 89
 0 87    104 148240     40 2798024    0    0 201800     0 5064 10857  1 11  0 89
 0 87    104 148684     40 2825632    0    0 206564    20 5244 9451  0 10  0 90
 2 86    104 149060     40 2899464    0    0 182584     0 6610 15984  0 10  0 90
 0 86    104 148004     40 2925044    0    0 167044     0 5725 13011  0 13  0 87
 0 84    104 148344     40 3025508    0    0 149640     0 6860 18340  0  9  0 91
 0 81    104 146152     40 3074460    0    0 167780     0 7572 19434  1 16  0 84
 0 79    104 149172     40 3127748    0    0 191064     0 5373 11928  0 10  0 90
 0 77    104 146972     40 3101440    0    0 135968     0 9232 28208  1 18  0 82
 0 70    104 148896     40 3081860    0    0 144372     0 7790 22857  0 16  0 84
 0 69    104 148492     40 3173532    0    0 154024     0 8002 22211  0 10  0 90
 1 68    104 145804     40 3192788    0    0 111412     0 9612 30604  0 14  0 86
 0 66    104 147324     40 3173004    0    0 137800     0 9858 31132  0 13  0 87
 0 65    104 145456     40 3121436    0    0 141348     0 6521 17443  1 13  0 87
 1 64    104 148408     40 3154056    0    0 149308     0 9575 29755  0 13  0 87
 0 61    104 145396     40 3187312    0    0 161976     0 9709 29021  0 13  0 87
 0 61    104 147860     40 3189972    0    0 154844     0 7345 20033  0 10  0 90
 2 61    104 148264     40 3162276    0    0 135804     0 10865 33643  0 15  1 8
5
 0 59    104 147264     40 3203068    0    0 70464     0 9340 29345  0 11  0 89
 1 58    104 147344     40 3203468    0    0 108072    48 12206 39011  0 15  0 8
5
 0 58    104 146696     40 3204660    0    0 102492    32 12800 43578  0 15  0 8
5
 1 58    104 145712     40 3201916    0    0 198728     8 10537 34621  0 15  0 8
5
 2 56    104 148732     40 3195160    0    0 119700     0 10647 35077  1 15  0 8
4
 0 54    104 148628     40 3203856    0    0 92588     0 8757 26257  0 12  0 88
 0 54    104 147128     40 3203956    0    0 207224     0 9706 29997  0 14  0 86
 1 54    104 146788     40 3165980    0    0 150120    36 10611 36721  0 17  0 8
3
 1 52    104 146984     40 3121696    0    0 160080     8 8714 28264  0 14  0 86
 0 51    104 147556     40 3123932    0    0 223940     0 6088 15013  0 13  0 87
 0 50    104 147268     40 3172724    0    0 154592     0 8949 26645  0 13  0 87
 0 46    104 148692     40 3135296    0    0 153708     0 7146 19844  1 15  0 85
 0 45    104 148116     40 3168180    0    0 165740    20 7884 22111  0 14  0 86
 0 43    104 147388     40 3209160    0    0 179588     8 9244 27501  0 14  0 86
 0 41    104 147760     40 3208860    0    0 131216    24 8205 24867  0 10  0 90
 0 41    104 148868     40 3122048    0    0 188260     0 9812 30872  0 14  0 86
 1 40    104 146832     40 3210452    0    0 168388     0 8798 27378  0 12  0 88
 1 39    104 146388     40 3211436    0    0 96044     0 13197 45162  0 15  2 83
 0 37    104 144716     40 3213412    0    0 151044    12 9877 30263  1 14  0 85
 0 37    104 146056     40 3212088    0    0 95988    27 13179 44933  0 15  5 80
 3 36    104 146776     40 3211552    0    0 66580     0 13651 46636  0 16  9 76
 1 37    104 146504     40 3212192    0    0 77196     0 16353 57091  0 18  0 82
 1 37    104 146864     40 3211648    0    0 135312     0 13350 45859  0 17  1 8
3
 2 35    104 148924     40 3209796    0    0 81228     0 13459 43748  0 15  0 85
 1 36    104 147792     40 3211332    0    0 107248     0 15935 57007  0 21  0 7
9
 2 35    104 146916     40 3212444    0    0 84328     0 16471 57611  0 18  0 82
 0 35    104 146500     40 3212944    0    0 98524     0 16049 55775  0 19  0 81
 0 34    104 148448     40 3211220    0    0 114116     0 14166 51140  1 17  2 8
1
 0 32    104 145220     40 3215024    0    0 96068     0 16008 58012  0 21  0 79
 1 29    104 145376     40 3215416    0    0 102604     0 12294 41704  0 16  1 8
4
 2 27    104 146764     40 3214384    0    0 132004     0 13076 44026  1 15  3 8
2
 0 27    104 148888     40 3212460    0    0 118908     0 16100 57546  0 19  0 8
1
 0 26    104 144932     40 3216260    0    0 101220    24 15687 55236  0 20  2 7
8
 1 24    104 147848     40 3214344    0    0 125152     8 13737 48996  0 19  0 8
1
 2 21    104 147024     40 3215676    0    0 71300     0 18828 65151  0 22  0 78
 1 22    104 145124     40 3217852    0    0 85228     0 19031 65807  1 20  0 80
 2 21    104 147472     40 3215532    0    0 96188     0 16644 60505  0 20  0 80
 2 18    104 146064     40 3217544    0    0 83316     0 16788 60762  1 21  0 79
 6 13    104 145824     40 3218832    0    0 56188     0 15100 51145  0 16  1 83
 0  6    104 147988     40 3218048    0    0 75836    20 19925 69456  0 22  0 78
 0  0    104 148592     40 3218292    0    0 20328     0 6190 17793  0  6 70 24
 0  0    104 148796     40 3218392    0    0    36     0 1199  372  1  0 98  1
 0  0    104 148796     40 3218392    0    0     0     0 1090  192  0  0 100  0
 0  0    104 149044     40 3218420    0    0    16     0 1110  349  0  0 100  0
 0  0    104 149044     40 3218420    0    0     0    17 1105  212  0  0 100  0
 0  0    104 149308     40 3218436    0    0     0     0 1143  317  0  0 100  0


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