On Wed, Nov 29, 2017 at 03:32:08PM +0900, Joonsoo Kim wrote: > On Thu, Nov 23, 2017 at 02:08:43PM +0000, Mel Gorman wrote: > > > 3. Another reason a linear scanner was used was because we wanted to > > clear entire pageblocks we were migrating from and pack the target > > pageblocks as much as possible. This was to reduce the amount of > > migration required overall even though the scanning hurts. This patch > > takes MIGRATE_MOVABLE pages from anywhere that is "not this pageblock". > > Those potentially have to be moved again and again trying to randomly > > fill a MIGRATE_MOVABLE block. Have you considered using the freelists > > as a hint? i.e. take a page from the freelist, then isolate all free > > pages in the same pageblock as migration targets? That would preserve > > the "packing property" of the linear scanner. > > > > This would increase the amount of scanning but that *might* be offset by > > the number of migrations the workload does overall. Note that migrations > > potentially are minor faults so if we do too many migrations, your > > workload may suffer. > > > > 4. One problem the linear scanner avoids is that a migration target is > > subsequently used as a migration source and leads to a ping-pong effect. > > I don't know how bad this is in practice or even if it's a problem at > > all but it was a concern at the time > > IIUC, this potential advantage for a linear scanner would not be the > actual advantage in the *running* system. > > Consider about following worst case scenario for "direct freepage > allocation" that "moved again" happens. > The immediate case should be ok as long as the migration source and the pageblock a freepage is taken from is not the same pageblock. That might mean that more pages from the freelist would need to be examined until another pageblock was found. > > So, I think that "direct freepage allocation" doesn't suffer from such > a ping-poing effect. Am I missing something? > The ping-pong effect I'm concerned with is that a previous migration target is used as a migration source in the future. It's hard for that situation to occur with two linear scanners but care is needed when using direct freepage allocation. -- Mel Gorman SUSE Labs -- 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>
