Lustre discuss - Dec 2008 - Lustre Intelligence?

So question, had a user, thought his problem was disk system, turns  
out he was just OOM machine, his IO code though looked like this:

   for(i=0;i<kloc;i++)
   {
     sprintf(&(buffer[39]),"%d",
                  i+(int)floor(rank*(double)k/(double)numprocs)+1);
     f1 = fopen(buffer,"w");
     for(j=0;j<N;j++)
     {
       fprintf(f1,"%e\n",u[i][j]);
     }
     fclose(f1);
   }


So how I read this every processor (every processor calls this  
function and writes to its own sets of files)  is writing one double  
at a time to their files.  IO performance though was still quite good.

I enabled extents_stats on rank0 of this job and ran it,  Here Is  
what I ended up with (stats were zeroed, and only job running on client)

       extents            calls    % cum%  |          calls    % cum%
    0K -    4K :                  12    4    4  |              0     
0    0
    4K -    8K :                    0    0    4  |              0     
0    0
    8K -   16K :                   0    0    4  |              0     
0    0
   16K -   32K :                  0    0    4  |              0     
0    0
   32K -   64K :                  0    0    4  |              0     
0    0
   64K -  128K :                 0    0    4  |              0    0    0
  128K -  256K :                0    0    4  |              0    0    0
  256K -  512K :                0    0    4  |              0    0    0
  512K - 1024K :              0    0    4  |              4    1    1
    1M -    2M :            136   47   51  |            220   98  100
    2M -    4M :            140   48  100  |              0    0  100


So 98% of writes and reads (read code is similar and reads in about  
2GB this way)  were all 1-4MB.  Is this lustre showing its''  
preference for 1MB IO ops? Even though the code wanted to do 8bytes  
at a time, lustre cleaned it up?   Or did LInux do this some place?

Brock Palen
www.umich.edu/~brockp
Center for Advanced Computing
brockp at umich.edu
(734)936-1985