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. __M1___F1_________________F2__F3___ M: migration source page (the number denotes the ID of the page) F: freepage (the number denotes the sequence in the freelist of the buddy) _: other pages migration scanner: move from left to right. If migration happens with "direct freepage allocation", memory state will be changed to: __F?___M1_________________F2__F3___ And then, please assume that there is an another movable allocation before another migration. It's reasonable assumption since there are really many movable allocations in the *running* system. __F?___M1_________________M2__F3___ If migration happens again, memory state will be: __F?___F?_________________M2__M1___ M1 is moved twice and overall number of migration is two. Now, think about a linear scanner. With the same scenario, memory state will be as following. __M1___F1_________________F2__F3___ __F?___F1_________________F2__M1___ __F?___M2_________________F2__M1___ __F?___F?_________________M2__M1___ Although "move again" doesn't happen in a linear scanner, final memory state is the same and the same number of migration happens. So, I think that "direct freepage allocation" doesn't suffer from such a ping-poing effect. Am I missing something? Thanks. -- 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>
