ceph/src/common/PriorityCache.cc

   1 // -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
   2 // vim: ts=8 sw=2 smarttab
   3 /*
   4  * Ceph - scalable distributed file system
   5  *
   6  * Copyright (C) 2018 Red Hat
   7  *
   8  * This is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License version 2.1, as published by the Free Software
  11  * Foundation.  See file COPYING.
  12  *
  13  */
  14
  15 #include "PriorityCache.h"
  16
  17 namespace PriorityCache {
  18   int64_t get_chunk(uint64_t usage, uint64_t total_bytes) {
  19     uint64_t chunk = total_bytes;
  20
  21     // Find the nearest power of 2
  22     chunk -= 1;
  23     chunk |= chunk >> 1;
  24     chunk |= chunk >> 2;
  25     chunk |= chunk >> 4;
  26     chunk |= chunk >> 8;
  27     chunk |= chunk >> 16;
  28     chunk |= chunk >> 32;
  29     chunk += 1;
  30     // shrink it to 1/256 of the rounded up cache size
  31     chunk /= 256;
  32
  33     // bound the chunk size to be between 4MB and 32MB
  34     chunk = (chunk > 4ul*1024*1024) ? chunk : 4ul*1024*1024;
  35     chunk = (chunk < 16ul*1024*1024) ? chunk : 16ul*1024*1024;
  36
  37     /* Add 16 chunks of headroom and round up to the near chunk.  Note that
  38      * if RocksDB is used, it's a good idea to have N MB of headroom where
  39      * N is the target_file_size_base value.  RocksDB will read SST files
  40      * into the block cache during compaction which potentially can force out
  41      * all existing cached data.  Once compaction is finished, the SST data is
  42      * released leaving an empty cache.  Having enough headroom to absorb
  43      * compaction reads allows the kv cache grow even during extremely heavy
  44      * compaction workloads.
  45      */
  46     uint64_t val = usage + (16 * chunk);
  47     uint64_t r = (val) % chunk;
  48     if (r > 0)
  49       val = val + chunk - r;
  50     return val;
  51   }
  52
  53   PriCache::~PriCache() {
  54   }
  55 }