Re: Interesting performance quirk.

[Colin Paul Gloster <Colin_Paul_Gloster@ACM.org>]

On Thu, 30 Oct 2008, Peter C. Chapin wrote:

|------------------------------------------------------------------------|
|"Colin Paul Gloster wrote:                                              |
|                                                                        |
|> Earlier this year I had used QEMU on Windows (possibly not Windows XP)|
|> to have a GNU/Linux distribution (possibly RedHat) emulated. I ran a  |
|> Bourne shell script or a Bourne Again Shell script in the emulated    |
|> system which made thousands of fairly short I/O transactions. The     |
|> emulated system including its pretend harddisk were kept small enough |
|> (no more than a few hundred megabytes) to be kept solely in the real  |
|> physical primary memory instead of relying on virtual memory.         |
|>                                                                       |
|> It was faster than running the same script on Cygwin on the same      |
|> machine.                                                              |
|                                                                        |
|That's interesting. I think it's probably conventional wisdom that doing|
|I/O in a VM would be slower than outside the virtual machine. I'm sure  |
|that's true in many cases, although the situation you described shows   |
|that it's not always true."                                             |
|------------------------------------------------------------------------|

For clarity, I explain that the I/O of the virtual machine which I
referred to was merely I/O to its emulated filesystems, all of which
together plus the emulated memory were small enough to fit into the
genuine physical memory of the host operating system.

|------------------------------------------------------------------------|
|"My program [..] the memory                                             |
|block I work over is only 1 MB long so I don't think paging would be an |
|issue (there is no disk activity when I run it). [..]                   |
|                                                                        |
|[..]"                                                                   |
|------------------------------------------------------------------------|

Your program's memory block's size might be of the order of one
megabyte, but I do not know whether the emulated filesystems which you
used were also small enough to fit into emulated memory. However, this
does not explain why one program you have tried has been sped up by
emulation whereas another has not been sped up.