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Re: concurrent direct IO write in xfs

To: Linda Walsh <xfs@xxxxxxxxx>
Subject: Re: concurrent direct IO write in xfs
From: Zheng Da <zhengda1936@xxxxxxxxx>
Date: Fri, 20 Jan 2012 10:07:01 -0500
Cc: Linux-Xfs <linux-xfs@xxxxxxxxxxx>
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Helllo

On Fri, Jan 20, 2012 at 3:53 AM, Linda Walsh <xfs@xxxxxxxxx> wrote:


Zheng Da wrote:

I create a file of 4GB in XFS (the ramdisk has 5GB of space). My test program overwrites 4G of data to the file  
----
       It sounds like you are asking why multiple threads don't
move memory from one point to another point in memory at a faster rate
than one thread alone.

I.e. if you had 2 processes doing an assembly instruction, memmov to move
a chunk of memory from 1 area to another, would you expect to do the move
any faster if you had 2 processors doing the move vs. 1??
Yes. Actually, for reading, using multiple threads is faster than a single thread. 
If you try simple memory copy with memcpy in the C library, the overall throughput will still increase if you use multiple processors.

I think the limiting factor (unless you have a slow processor and some
REALLY fast memory, but stock x86-64 parts, today have memory running about
2-4 times slower than the processor -- so the memory is usually the bottleneck.
Memory bandwidth will eventually become a bottleneck, but it can still scale for a small number of processors. 

Two processes wouldn't do it any faster, and might actually do it slower due to
resource contention issues -- I would *think*... but I really don't know the
details of how writing from mem2mem and having the target be in the format of
and xfs file system, would cause cpu-bound delays that would be significant to
change the fact that m2m operations are usually mem-bandwidth limited...?

(I don't know the answers, just clarifying what you are asking)...

Da 

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