how to knock a page out of file cache

[birdsong]

Can anyone recommend a way to read a file ensuring that it is not
coming from file cache on Linux?

I'm trying to write a metric script for measuring http connect + read
times from a web server over localhost. I want to plot both file
cache read times and non-cached files.

I thought of simply opening and writing to the file to dirty it's
pages, but there no guarantee that pdflush will have already written
the dirty pages to disk -pretty sure it depends on all the dirty ratio
and intervals.

Does anybody know of a system call that will 'knock' the file out of
file cache? Can madvise or fadvise do this?

I know this is mostly a unix question, but I'm asking in the context
of how to do this in python.

 

[Paul Rubin]

Does anybody know of a system call that will 'knock' the file out of
file cache? Can madvise or fadvise do this?

I don't think so. Best I can think of is unmount and remount the file
system. And I don't know if even that is guaranteed if you don't
physically remove/unplug the device.

 

[Ishwor Gurung]

[...]

I thought of simply opening and writing to the file to dirty it's
pages, but there no guarantee that pdflush will have already written
the dirty pages to disk -pretty sure it depends on all the dirty ratio
and intervals.

Does anybody know of a system call that will 'knock' the file out of
file cache?  Can madvise or fadvise do this?

Does this help http://www.westnet.com/~gsmith/content/linux-pdflush.htm ?
(Getting hold of procfs and doing it in Python?)

Also, are you looking for sync(2) http://linux.die.net/man/2/sync?

[...]

 

[Ishwor Gurung]

[...]

Also, are you looking for sync(2) http://linux.die.net/man/2/sync?

Also, maybe mmap.flush([offset, size]) @
http://docs.python.org/library/mmap.html ?
Get the file, mmap it and flush it. The docs has more info on flush(...)

Afaik, ultimately the kernel will control the writebacks of dirty
pages and their frequency of writebacks.
So if you want the shortest path, you can choose to go the /proc path.

[...]