title: redis bgsave failed because fork Cannot allocate memory
body: <p>all:
  here is my server memory info with 'free -m'</p>
  
  <pre><code>              total       used       free     shared    buffers     cached
   Mem:         64433       49259      15174          0          3         31
   -/+ buffers/cache:      49224      15209
   Swap:         8197        184       8012
  </code></pre>
  
  <p>my redis-server has used 46G memory, there is almost 15G memory left free</p>
  
  <p>As my knowledge,fork is copy on write, it should not failed when there has 15G free memory,which is enough to malloc necessary kernel structures .</p>
  
  <p>besides, when redis-server used 42G memory, bgsave is ok and fork is ok too.</p>
  
  <p>Is there any vm parameter I can tune to make fork return success ?</p>