Hello, inspired by recent reports [1] of problems with mbcache I had a look into what we could to improve it. I found the current code rather overengineered (counting with single entry being in several indices, having homegrown implementation of rw semaphore, ...). After some thinking I've decided to just reimplement mbcache instead of improving the original code in small steps since the fundamental changes in locking and layout would be actually harder to review in small steps than in one big chunk and overall the new mbcache is actually pretty simple piece of code (~450 lines). The result of rewrite is smaller code (almost half the original size), smaller cache entries (7 longs instead of 13), and better performance (see below for details). For measuring performance of mbcache I have written a stress test (I called it xattr-bench). The test spawns P processes, each process sets xattr for F different files, and the value of xattr is randomly chosen from a pool of V values. Each process runs until it sets extended attribute 50000 times (this is arbitrarily chosen number so that run times for the particular test machine are reasonable). The test machine has 24 CPUs and 64 GB of RAM, the test filesystem was created on ramdisk. Each test has been run 5 times. I have measured performance for original mbache, new mbcache2 code where LRU is implemented as a simple list, mbcache2 where LRU is implemented using list_lru, and mbcache2 where we keep LRU lazily and just use referenced bit. I have also measured performance when mbcache was completely disabled (to be able to quantify how much gain can some loads get from disabling mbcache). The graphs for different combinations of parameters (I have measured P=1,2,4,8,16,32,64; F=10,100,1000; V=10,100,1000,10000,100000) can be found at [2]. Based on the numbers I have chosen the implementation using LRU with referenced bit for submission. Implementation using list_lru is faster in some heavily contended cases but slower in most of the cases so I figured it is not worth it. My measurements show that completely disabling mbcache can still result in upto ~2x faster execution of the benchmark so even after improvements there is some gain users like Lustre or Ceph could have from completely disabling mbcache. Here is a comparison table with averages of 5 runs. Measured numbers are in order "old mbcache", "mbcache2 with normal LRU", "mbcache2 with list_lru LRU", "mbcache2 with referenced bit", "disabled mbcache". Note that some numbers for "old mbcache" are not available since the machine just dies due to softlockups under the pressure. V=10 F\P 1 2 4 8 16 32 64 10 0.158,0.157,0.209,0.155,0.135 0.208,0.196,0.263,0.229,0.154 0.500,0.277,0.364,0.305,0.176 0.798,0.400,0.380,0.384,0.237 3.258,0.584,0.593,0.664,0.500 13.807,1.047,1.029,1.100,0.986 61.339,2.803,3.615,2.994,1.799 100 0.172,0.167,0.161,0.185,0.126 0.279,0.222,0.244,0.222,0.156 0.520,0.275,0.275,0.273,0.199 0.825,0.341,0.408,0.333,0.217 2.981,0.505,0.523,0.523,0.315 12.022,1.202,1.210,1.125,1.293 44.641,2.943,2.869,3.337,13.056 1000 0.185,0.174,0.187,0.153,0.160 0.297,0.239,0.247,0.227,0.176 0.445,0.283,0.276,0.272,0.957 0.767,0.340,0.357,0.324,1.975 2.329,0.480,0.498,0.476,5.391 6.342,1.198,1.235,1.204,8.283 16.440,3.888,3.817,3.896,17.878 V=100 F\P 1 2 4 8 16 32 64 10 0.162,0.153,0.180,0.126,0.126 0.200,0.186,0.241,0.165,0.154 0.362,0.257,0.313,0.208,0.181 0.671,0.496,0.422,0.379,0.194 1.433,0.943,0.773,0.676,0.570 3.801,1.345,1.353,1.221,1.021 7.938,2.501,2.700,2.484,1.790 100 0.153,0.160,0.164,0.130,0.144 0.221,0.199,0.232,0.217,0.166 0.404,0.264,0.300,0.270,0.180 0.945,0.379,0.400,0.322,0.240 1.556,0.485,0.512,0.496,0.339 3.761,1.156,1.214,1.197,1.301 7.901,2.484,2.508,2.526,13.039 1000 0.215,0.191,0.205,0.212,0.156 0.303,0.246,0.246,0.247,0.182 0.471,0.288,0.305,0.300,0.896 0.960,0.347,0.375,0.347,1.892 1.647,0.479,0.530,0.509,4.744 3.916,1.176,1.288,1.205,8.300 8.058,3.160,3.232,3.200,17.616 V=1000 F\P 1 2 4 8 16 32 64 10 0.151,0.129,0.179,0.160,0.130 0.210,0.163,0.248,0.193,0.155 0.326,0.245,0.313,0.204,0.191 0.685,0.521,0.493,0.365,0.210 1.284,0.859,0.772,0.613,0.389 3.087,2.251,1.307,1.745,0.896 6.451,4.801,2.693,3.736,1.806 100 0.154,0.153,0.156,0.159,0.120 0.211,0.191,0.232,0.194,0.158 0.276,0.282,0.286,0.228,0.170 0.687,0.506,0.496,0.400,0.259 1.202,0.877,0.712,0.632,0.326 3.259,1.954,1.564,1.336,1.255 8.738,2.887,14.421,3.111,13.175 1000 0.145,0.179,0.184,0.175,0.156 0.202,0.222,0.218,0.220,0.174 0.449,0.319,0.836,0.276,0.965 0.899,0.333,0.793,0.353,2.002 1.577,0.524,0.529,0.523,4.676 4.221,1.240,1.280,1.281,8.371 9.782,3.579,3.605,3.585,17.425 V=10000 F\P 1 2 4 8 16 32 64 10 0.161,0.154,0.204,0.158,0.137 0.198,0.190,0.271,0.190,0.153 0.296,0.256,0.340,0.229,0.164 0.662,0.480,0.475,0.368,0.239 1.192,0.818,0.785,0.646,0.349 2.989,2.200,1.237,1.725,0.961 6.362,4.746,2.666,3.718,1.793 100 0.176,0.174,0.136,0.155,0.123 0.236,0.203,0.202,0.188,0.165 0.326,0.255,0.267,0.241,0.182 0.696,0.511,0.415,0.387,0.213 1.183,0.855,0.679,0.689,0.330 4.205,3.444,1.444,2.760,1.249 19.510,17.760,15.203,17.387,12.828 1000 0.199,0.183,0.183,0.183,0.164 0.240,0.227,0.225,0.226,0.179 1.159,1.014,1.014,1.036,0.985 2.286,2.154,1.987,2.019,1.997 6.023,6.039,6.594,5.657,5.069 N/A,10.933,9.272,10.382,8.305 N/A,36.620,27.886,36.165,17.683 V=100000 F\P 1 2 4 8 16 32 64 10 0.171,0.162,0.220,0.163,0.143 0.204,0.198,0.272,0.192,0.154 0.285,0.230,0.318,0.218,0.172 0.692,0.500,0.505,0.367,0.210 1.225,0.881,0.827,0.687,0.338 2.990,2.243,1.266,1.696,0.942 6.379,4.771,2.609,3.722,1.778 100 0.151,0.171,0.176,0.171,0.153 0.220,0.210,0.226,0.201,0.167 0.295,0.255,0.265,0.242,0.175 0.720,0.518,0.417,0.387,0.221 1.226,0.844,0.689,0.672,0.343 3.423,2.831,1.392,2.370,1.354 19.234,17.544,15.419,16.700,13.172 1000 0.192,0.189,0.188,0.184,0.164 0.249,0.225,0.223,0.218,0.178 1.162,1.043,1.031,1.024,1.003 2.257,2.093,2.180,2.004,1.960 5.853,4.997,6.143,5.315,5.350 N/A,10.399,8.578,9.190,8.309 N/A,32.198,19.465,19.194,17.210 Thoughs, opinions, comments welcome. Honza [1] https://bugzilla.kernel.org/show_bug.cgi?id=107301 [2] http://beta.suse.com/private/jack/mbcache2/ -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
