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From Chris Collins <chris_j_coll...@yahoo.com>
Subject Re: Optimizing indexes with mulitiple processors?
Date Fri, 10 Jun 2005 15:46:43 GMT
Hi John, your comments are correct.  But based on the fact we know on our box
we have almost 80MB sustainable bandwidth and very low latency to disk per
second, and observing that the io we are doing in lucene is small in comparison
a 1 second I am reasonably confident that this time spent is not far out (for
this run).  

As I may have mentioned before, in my case we have reasonably fast hardware
raid.  At that point the bottlenecks of course change.  We also have the case
where we write to a filer which has good bandwidth but high latency.  Here we
see that merge is io bound as you would expect.  That’s why I assume changing
the buffer sizes of the FS streams could help assuming the merge operations
read and write to the segments in a linear fashion…in this case, the latency is
not really a function of the disks, but a function of the latency in the rpc
between the client (indexer) and the filer.  By increasing the buffer sizes we
would reduce the amount of RPC’s. 

>From an IO bound point of view one needs to consider if you have saturated the
device or you are just stuck waiting for the disk to rotate around.  Long gone
is escalator algorithm as the preferred disk optimization of seagate :-}, disk
can take many instructions and re-order them to minimize latency issues. If its
a latency issue and not necessarily bandwidth then using overlapping io can
improve throughput (splitting the index and having multiple writer threads
would give you that). In fact in my silly filer example having multiple writers
does show good effect.  Of course this depends on if you can finagle your
application to allow you to split the indices.

Further I have done longer runs to plot throughput over time (16M doc crawls). 
I only profiled 4k docs since I didn’t want to wait forever with JProbe.  Not
sure what the correct jargon is here so excuse my description.  The in memory
objects were merged out to disk but we didnt get the second order effect of the
maybeMerge function finding enough segments on that level to trigger the
merging of multiple segments for the next tier (segments * mergefactor). 
Indexer throughput is not of course constant, over time the time to index one
document does increase when you take into account the cost of the merges.  But
due to the pyramid effect of how the merger works, the larger order merges of
course happen less and less.  

Back to my observations.  From the CPU part of indexing, the inversion aspect
is dwarfed by the standard tokenizer.  My hat off to Doug (what is hogging the
cpu is auto generated code :-} )Given multiple cores / ht/ smp your certainly
can capitalize on them if you so wish to write the code.  Not all IO bound
problems are created equal, if it is merely latency then you still have room to
improve throughput if you massage your indexing approach.  Using a single
indexing thread and seeing your io bound should not be a reason to give up :-} 

As you can tell I have two indexing worlds, one where my disk is fast (CPU
Bound) and one where it is slow (IO Bound).  I have to capitalize on the
effects of both to get my job done and each of them have distinctive
challenges.

Regards

Chris
--- John Haxby <jch@scalix.com> wrote:

> Chris Collins wrote:
> 
> >Ok that part isnt surprising.  However only about 1% of 30% of the merge was
> >spent in the OS.flush call (not very IO bound at all with this controller).
> >  
> >
> On Linux, at least, measuring the time taken in OS.flush is not a good 
> way to determine if you're I/O bound -- all that does is transfer the 
> data to the kernel.   Later, possibly much later, the kernel will 
> actually write the data to the disk.
> 
> The upshot of this is that if the size of the index is around the size 
> of physical memory in the system, optimizing will appear CPU bound.   
> Once the index exceeds the size of physical memory, you'll see the 
> effects of I/O.   OS.flush will still probably be ver quick, but you'll 
> see a lot of I/O wait if you run, say, top.
> 
> jch
> 
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