On Mon, Feb 22, 2016 at 06:23:19PM -0800, David Rientjes wrote: > On Mon, 22 Feb 2016, Johannes Weiner wrote: > > > In machines with 140G of memory and enterprise flash storage, we have > > seen read and write bursts routinely exceed the kswapd watermarks and > > cause thundering herds in direct reclaim. Unfortunately, the only way > > to tune kswapd aggressiveness is through adjusting min_free_kbytes - > > the system's emergency reserves - which is entirely unrelated to the > > system's latency requirements. In order to get kswapd to maintain a > > 250M buffer of free memory, the emergency reserves need to be set to > > 1G. That is a lot of memory wasted for no good reason. > > > > On the other hand, it's reasonable to assume that allocation bursts > > and overall allocation concurrency scale with memory capacity, so it > > makes sense to make kswapd aggressiveness a function of that as well. > > > > Change the kswapd watermark scale factor from the currently fixed 25% > > of the tunable emergency reserve to a tunable 0.001% of memory. > > > > Making this tunable independent of min_free_kbytes is great. > > I'm wondering how the choice of 0.001% was picked for default? One of my > workstations currently has step sizes of about 0.0005% so this will be > doubling the steps from min to low and low to high. I'm not objecting to > that since it's definitely in the right direction (more free memory) but I > wonder if it will make a difference for some users. I wish it were a bit more scientific, but I basically picked an order of magnitude that sounds like a reasonable balance between wasted memory and expected allocation bursts before kswapd can ramp up. On a 10G machine, a 10M latency buffer sounds adequate, whereas 1M might get overwhelmed and 100M is almost certainly a waste of RAM. > > Beyond 1G of memory, this will produce bigger watermark steps than the > > current formula in default settings. Ensure that the new formula never > > chooses steps smaller than that, i.e. 25% of the emergency reserve. > > > > On a 140G machine, this raises the default watermark steps - the > > distance between min and low, and low and high - from 16M to 143M. > > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > Acked-by: Mel Gorman <mgorman@xxxxxxx> > > --- > > Documentation/sysctl/vm.txt | 18 ++++++++++++++++++ > > include/linux/mm.h | 1 + > > include/linux/mmzone.h | 2 ++ > > kernel/sysctl.c | 10 ++++++++++ > > mm/page_alloc.c | 29 +++++++++++++++++++++++++++-- > > 5 files changed, 58 insertions(+), 2 deletions(-) > > > > v2: Ensure 25% of emergency reserves as a minimum on small machines -Rik > > > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt > > index 89a887c..b02d940 100644 > > --- a/Documentation/sysctl/vm.txt > > +++ b/Documentation/sysctl/vm.txt > > @@ -803,6 +803,24 @@ performance impact. Reclaim code needs to take various locks to find freeable > > directory and inode objects. With vfs_cache_pressure=1000, it will look for > > ten times more freeable objects than there are. > > > > +============================================================= > > + > > +watermark_scale_factor: > > + > > +This factor controls the aggressiveness of kswapd. It defines the > > +amount of memory left in a node/system before kswapd is woken up and > > +how much memory needs to be free before kswapd goes back to sleep. > > + > > +The unit is in fractions of 10,000. The default value of 10 means the > > +distances between watermarks are 0.001% of the available memory in the > > +node/system. The maximum value is 1000, or 10% of memory. > > + > > The effective maximum value can be different than the tunable, though, > correct? It seems like you'd want to document why watermark_scale_factor > and the actual watermarks in /proc/zoneinfo may be different on some > systems. You mean because of the enforced minimum? I wondered about that, but it seems more like an implementation detail rather than part of the API. I doubt that in practice anybody would intentionally set the scale factor low enough for the kernel minimum to kick in. -- 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>