On Thu, Mar 28, 2013 at 11:00:53PM -0300, Rafael Aquini wrote: > On Tue, Mar 26, 2013 at 01:37:06PM +0800, Shaohua Li wrote: > > I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% > > CPU time (when cluster is hard to find, the CPU time can be up to 80%), which > > becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page > > cluster, which is very slow. > > > > Here I introduced a simple algorithm to search cluster. Since we only care > > about 256 pages cluster, we can just use a counter to track if a cluster is > > free. Every 256 pages use one int to store the counter. If the counter of a > > cluster is 0, the cluster is free. All free clusters will be added to a list, > > so searching cluster is very efficient. With this, scap_swap_map() overhead > > disappears. > > > > Since searching cluster with a list is easy, we can easily implement a per-cpu > > cluster algorithm to do block allocation, which can make swapout more > > efficient. This is in my TODO list. > > > > This might help low end SD card swap too. Because if the cluster is aligned, SD > > firmware can do flash erase more efficiently. > > > > We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has > > downside with the algorithm which might introduce regression (see below). > > > > The patch slightly changes which cluster is choosen. It always adds free > > cluster to list tail. This can help wear leveling for low end SSD too. And if > > no cluster found, the scan_swap_map() will do search from the end of last > > cluster. So if no cluster found, the scan_swap_map() will do search from the > > end of last free cluster, which is random. For SSD, this isn't a problem at > > all. > > > > Another downside is the cluster must be aligned to 256 pages, which will reduce > > the chance to find a cluster. I would expect this isn't a big problem for SSD > > because of the non-seek penality. (And this is the reason I only enable the > > algorithm for SSD). > > > > V3 -> V4: > > clean up > > > > V2 -> V3: > > rebase to latest linux-next > > > > V1 -> V2: > > 1. free cluster is added to a list, which makes searching cluster more efficient > > 2. only enable the algorithm for SSD. > > > > Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx> > > --- > > Hello Shaohua, > > I'm still OK with your series and tests I've been making with it are going fine > and dandy. I guess there were a couple of questions Andrew has raised you left > yet opened, but I'm assuming you both might have sorted them out in private. > > If you're going to resubmit this work to make any extra adjustment, please > consider the suggestions that follow below. (mostly cosmetics changes/nitpicks) Nice, makes the description quite clear, Thanks! I'd like to add these for an add-on cleanup patch if Andrew accepts the patches. Lazy to repost again :) Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>