On 07/29/2007 05:20 PM, Ray Lee wrote:
I understand what log structure is generally, but how does it help swapin?
Look at the swap out case first.
Right now, when swapping out the kernel places whatever it can
wherever it can inside the swap space. The closer you are to filling
your swap space, the more likely that those swapped out blocks will be
all over place, rather than in one nice chunk. Contrast that with a
log structured scheme, where the writeout happens to sequential spaces
on the drive instead of scattered about.
This seems to be now fixing the different problem of swap-space filling up.
I'm quite willing to for now assume I've got plenty free.
So, at some point when the system needs to fault those blocks that
back in, it now has a linear span of sectors to read instead of asking
the drive to bounce over twenty tracks for a hundred blocks.
Moreover though -- what I know about log structure is that generally it
optimises for write (swapout) and might make read (swapin) worse due to
fragmentation that wouldn't happen with a regular fs structure.
I guess that cleaner that Alan mentioned might be involved there -- I don't
know how/what it would be doing.
So, it eliminates the seeks. My laptop drive can read (huh, how odd,
it got slower, need to retest in single user mode), hmm, let's go with
about 25 MB/s. If we ask for a single block from each track, though,
that'll drop to 4k * (1 second / seek time) which is about a megabyte
a second if we're lucky enough to read from consecutive tracks. Even
worse if it's not.
Seeks are the enemy any time you need to hit the drive for anything,
be it swapping or optimizing a database.
I am very aware of the costs of seeks (on current magnetic media).
Rene.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
