Messages in this thread | | | Date | Sun, 29 Jul 2007 17:36:44 +0200 | From | Rene Herman <> | Subject | Re: RFT: updatedb "morning after" problem [was: Re: -mm merge plans for 2.6.23] |
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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.
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