On 01/28/2011 11:28 AM, Andrea Arcangeli wrote:
In short I think the zone balancing problem tackled in kswapd is wrong and kswapd should stick to the high wmark only, and if you care about zone balancing it should be done in the allocator only, then kswapd will cope with whatever the allocator decides just fine.
The allocator does not have information on which memory zones have more heavily used data vs which zones have less frequently used data. When the system starts up, we do our initial allocations in the top zone. This includes both heavily used files (like libc) and never-used-again files, as well as daemons that are active and daemons that go to sleep and never do anything again. After initial startup, we may eventually end up falling back to lower memory zones. In short, we may have an imbalance between the zones in how actively memory is used, from the moment the system has started up. The distance between the low and high watermarks corresponds only to the relative size of each zone. Having kswapd move only between these two watermarks means that memory in each zone is allocated and freed only according to zone size, not according to how actively used the memory in each zone is. Giving kswapd a little bit of extra room where it is allowed to extra free pages in a zone with lots of infrequently used and easily reclaimable pages, when another zone in the same node suffers from harder to deal with memory pressure, will steer more allocations towards the memory zone that has less pressure. This should even out the pressure between zones over time. We have had the kernel work like this since 2.6.0, and I believe that removing this "pressure valve" from the VM will result in the kind of balancing problems we had in some 2.4 kernels. Reducing the size of the gap is fine with me, since the pressure should even out over time. Removing the gap is just asking for trouble. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>