Sort Files Merge

[Gil Shomron]

Hi all,

I've been investigating sort functionality when sorting big files 
(larger than RAM size). I'm looking at the system calls and the actual 
read and write related instructions the application uses.
For sort to manage large files, my guess is, it implements some kind of 
external merge sort algorithm. And indeed, I can see sort load chunks 
from the big file, sorts them, and saves them back to disk when sorted.

But looking a bit further there are two things I do not understand:
1) There are plenty of reads to different buffers of different sizes 
(49152, 36864, 28672 for example) - why? I mean, why it doesn't just 
read the whole chunk once and then merge sort it?
2) After splitting and sorting each chunk, sort needs to merge them to 
one sorted list. When I examine the instructions it seems like sort does 
A LOT of accesses to those buffers in order to sort them, almost at the 
same intensity of merging each chunk - why? doesn't it just need to look 
at the beginning of each file, check where is the smallest value, put it 
in an output buffer and then write it to the final output file?

I've been digging in the code, but unfortunately didn't find my answers 
there yet.

I will really appreciate your help with helping me understand how sort 
is really implemented.

Thanks!
   Gil Shomron.