Perl: write speed mystery?

Answer 1

All data is cached in buffers before being effectively layed in the physical disk. A buffer from the system, another inside the disk itself (a 32MB buffer probably). While you fill those buffers, your program run at full speed and 100% CPU. Once the buffers are full, your program get to wait for the disk, which is much, much slower than memory and buffers, and this wait makes you stop consuming all this CPU.

Maybe you can make your code "wait for the disk" from the start, by using some Perl equivalent to fflush().

Answer 2

Maybe the OS is writing to disk as fast as it can (85 MB/s), and is putting the excess 35 MB/s in a buffer, and when it fills, is pausing the app to flush the buffer. Since the buffer is drained at 85 MB/s, you'd expect it to take 35/85 = ~0.4 times as long to drain as to fill. That's broadly compatible with your graph, if i squint enough.

You can estimate the size of the buffer as the product of the pause time and the disk speed.

Answer 3

Look at the graph! The green line indicates the average disk queue length. At one moment, it gets a peak and the CPU goes to 0 afterwards. IO Writes also goes to 0. It goes back to normal until a second peak is shown. Then CPU and IO writes return to normal. Then both IO and CPU drop again, to go up again at the next Queue peak. And again down, then up again...

It could be that the disk is doing the physical writes at that moment. However, it could also be that the system is doing a disk validation at that moment, reading the dat it just wrote to validate the writes, making sure the data is written correctly.

Another thing I notice is the 2.7 GB size. Since you're running this on a Windows system, I become a bit suspicious since that's about the amount of memory that Windows can handle, as a 32-bits process. The 64-bits Windows will provide the application up to 3 GB of RAM (a bit less) but then it needs to release it again. You might want to use Process Explorer to check the amount of RAM in use and the amount of IO reads.

And perhaps use a 64-bits Perl version...

Answer 4

I am with everyone else who is saying that the problem is buffers filling and then emptying. Try turning on autoflush to avoid having a buffer (in Perl):

#!/usr/bin/perl

use strict;
use warnings;

use IO::Handle;

my $filename = "output.txt";

open my $numbers_outfile, ">", $filename
    or die "could not open $filename: $!";

$numbers_outfile->autoflush(1);

#each time through the loop should be 1 gig
for (1 .. 20) {
    #each time though the loop should be 1 meg
    for (1 .. 1024) {
        #print 1 meg of Zs
        print {$numbers_outfile} "Z" x (1024*1024)
    }
}

Buffers can be good if you are going to print a little, do so work, print a litte, do some work, etc. But if you are just going to be blasting data onto disk, they can cause odd behavior. You may also need to disable any write caching your filesystem is doing.