RE: why swap at all?

[Buddy Lumpkin]

> Executables and shared libraries live in the filesystem
> cache.  Evicting those from memory - because swapping is
> disabled and "the VM should remove something from cache
> instead" - will feel exactly the same as swapping ...

I totally agree, you can't get away from evicting pages from memory to disk
if your doing file system I/O because you eventually fill up memory. Any
additional file system I/O requires evictions, period.

Trying to preference executables and shared libraries is difficult because
they are backed by named files, hence they also pagecache. (kind of reminds
me of that little white speck on chicken poop. If it came out of the
chicken, it's chicken poop too :)

But there is the case where massive amounts of file system I/O (consider
several fibre cards connecting to SAN attached storage that saturates the
centerplane on some insanely large system) will force pages from running
executables to be evicted, only to be faulted back a few milliseconds later.
It's this thrashing effect that a separation eliminates if you have the
ability to distinguish between executables (not just files mapped in
executable but actual running processes) and non-executable pages.

Consider how silly it would be for a system running a single process that
consumes only 100k that generates so much filesystem I/O that the process is
constantly paged out. When it needs to wake up again, becomes runnable and
the program counter starts to access pages within the process address space,
it gets faulted back in.

Lather, Rinse, Repeat ...

--Buddy



-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/