On Fri, Jul 26, 2013 at 02:44:29PM +0200, Michal Hocko wrote: > I would like to resurrect Dave's patch. It was originally posted here > https://lkml.org/lkml/2010/9/16/250 and I have resurrected it here > https://lkml.org/lkml/2012/10/12/175 for the first time. There didn't > seem to be any strong opposition but the patch has been dropped later > from the mm tree. > > To summarize concerns: > Kosaki was worried about possible excessive logging when somebody drops > caches too often (but then he claimed he didn't have a strong opinion on > that) and later acked the patch (https://lkml.org/lkml/2012/10/12/350). > I would even dare to say opposite. If somebody drops caches too often > then I would really like to know that from the log when supporting a > system because it almost for sure means that there is something fishy > going on. It is also worth mentioning that only root can write drop > caches so this is not an flooding attack vector. Agreed. > Andrew was worried (http://lkml.indiana.edu/hypermail/linux/kernel/1210.3/00605.html) > about people hating us because they are using this as a solution to > their issues. I concur that most of those are just hacks that found > their way into scripts looong time agon and stayed there. We should > rather not feed these cargo cults and rather fix the real bugs. History > has been showing us that users are usually getting rid of old hacks when > something starts screeming at them. So let's screem. Agreed. The whole point of this is to be a pain in the ass in order to establish a feedback loop. > Boris then noted (http://lkml.indiana.edu/hypermail/linux/kernel/1210.3/00659.html) > that he is using drop_caches to make s2ram faster but as others noted > this just adds the overhead to the resume path so it might work only for > certain use cases. Having a low priority message under such conditions > shouldn't such a big deal. A oneliner like this should drown in the overall noise of the suspend-resume path. > I am bringing the patch up again because this has proved being really > helpful when chasing strange performance issues which (surprise > surprise) turn out to be related to artificially dropped caches done > because the admin thinks this would help... So mostly those who support > machines which are not in their hands would benefit from such a change. > > I have just refreshed the original patch on top of the current mm tree > and lowered priority to KERN_INFO to make the message less hysterical. > > : From: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> > : Date: Fri, 12 Oct 2012 14:30:54 +0200 > : > : There is plenty of anecdotal evidence and a load of blog posts > : suggesting that using "drop_caches" periodically keeps your system > : running in "tip top shape". Perhaps adding some kernel > : documentation will increase the amount of accurate data on its use. > : > : If we are not shrinking caches effectively, then we have real bugs. > : Using drop_caches will simply mask the bugs and make them harder > : to find, but certainly does not fix them, nor is it an appropriate > : "workaround" to limit the size of the caches. > : > : It's a great debugging tool, and is really handy for doing things > : like repeatable benchmark runs. So, add a bit more documentation > : about it, and add a little KERN_NOTICE. It should help developers > : who are chasing down reclaim-related bugs. > > [mhocko@xxxxxxx: refreshed to current -mm tree] > [akpm@xxxxxxxxxxxxxxxxxxxx: checkpatch fixes] > Signed-off-by: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> > Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- 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>