On Sep 24, 2008  16:35 -0400, Theodore Ts'o wrote:
quoted text
> On the other hand, if we take your iop/s and translate them to
> milliseconds so we can measure the latency in the case where the
> workload is essentialy doing random reads, and then cross correlated
> it with my measurements, we get this table:

Comparing the incremental benefit of each step:

quoted text
> i/o size iops/s  ms latency  % degredation         % improvement
>     	 	    	     of random inodes   of related inodes I/O
>    4k	  131       7.634      
>    8k	  130	    7.692	0.77%		    11.3%
                                       1.57%              10.5%
quoted text
>   16k	  128	    7.813	2.34%		    21.8%
                                       1.63%               7.8%
quoted text
>   32k	  126	    7.937	3.97%		    29.6%
                                       4.29%               5.9%
quoted text
>   64k	  121	    8.264	8.26%		    35.5%
                                       7.67%               4.5%
quoted text
>  128k	  113	    8.850      15.93%		    40.0%
                                      16.07%               2.4%
quoted text
>  256k	  100	   10.000      31.00%		    42.4%
> 
> Depending on whether you believe that workloads involving random inode
> reads are more common compared to related inodes I/O, the sweet spot
> is probably somewhere between 32k and 128k.  I'm open to opinions
> (preferably backed up with more benchmarks of likely workloads) of
> whether we should use a default value of inode_readahead_bits of 4 or
> 5 (i.e., 64k, my original guess, or 128k, in v2 of the patch).  But
> yes, making it tunable is definitely going to be necessary, since for
> different workloads (i.e squid vs. git repositories) will have very
> different requirements.

It looks like moving from 64kB to 128kB readahead might be a loss for
"unknown" workloads, since that increases latency by 7.67% for the random
inode case, but we only get 4.5% improvement in the sequential inode case.
Also recall that at large scale "htree" breaks down to random inode
lookup so that isn't exactly a fringe case (though readahead may still
help if the cache is large enough).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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