[Tutor] concurrent file reading using python

Steven D'Aprano steve at pearwood.info
Tue Mar 27 00:21:13 CEST 2012

Abhishek Pratap wrote:
> Hi Guys
> I want to utilize the power of cores on my server and read big files
> (> 50Gb) simultaneously by seeking to N locations.

Yes, you have many cores on the server. But how many hard drives is each file 
on? If all the files are on one disk, then you will *kill* performance dead by 
forcing the drive to seek backwards and forwards:

seek to 12345678
read a block
seek to 9947500
read a block
seek to 5891124
read a block
seek back to 12345678 + 1 block
read another block
seek back to 9947500 + 1 block
read another block

The drive will spend most of its time seeking instead of reading.

Even if you have multiple hard drives in a RAID array, performance will depend 
strongly the details of how it is configured (RAID1, RAID0, software RAID, 
hardware RAID, etc.) and how smart the controller is.

Chances are, though, that the controller won't be smart enough. Particularly 
if you have hardware RAID, which in my experience tends to be more expensive 
and less useful than software RAID (at least for Linux).

And what are you planning on doing with the files once you have read them? I 
don't know how much memory your server has got, but I'd be very surprised if 
you can fit the entire > 50 GB file in RAM at once. So you're going to read 
the files and merge the output... by writing them to the disk. Now you have 
the drive trying to read *and* write simultaneously.


Tasks which are limited by disk IO are not made faster by using a faster CPU, 
since the bottleneck is disk access, not CPU speed.

Back in the Ancient Days when tape was the only storage medium, there were a 
lot of programs optimised for slow IO. Unfortunately this is pretty much a 
lost art -- although disk access is thousands or tens of thousands of times 
slower than memory access, it is so much faster than tape that people don't 
seem to care much about optimising disk access.

> What I want to know is the best way to read a file concurrently. I
> have read about file-handle.seek(),  os.lseek() but not sure if thats
> the way to go. Any used cases would be of help.

Optimising concurrent disk access is a specialist field. You may be better off 
asking for help on the main Python list, comp.lang.python or 
python-list at python.org, and hope somebody has some experience with this. But 
chances are very high that you will need to search the web for forums 
dedicated to concurrent disk access, and translate from whatever language(s) 
they are using to Python.


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