Messages in this thread | | | Subject | Re: [PATCH] mm: thp: remove use_zero_page sysfs knob | From | Yang Shi <> | Date | Mon, 23 Jul 2018 14:49:34 -0700 |
| |
On 7/23/18 2:33 PM, David Rientjes wrote: > On Mon, 23 Jul 2018, David Rientjes wrote: > >>>> The huge zero page can be reclaimed under memory pressure and, if it is, >>>> it is attempted to be allocted again with gfp flags that attempt memory >>>> compaction that can become expensive. If we are constantly under memory >>>> pressure, it gets freed and reallocated millions of times always trying to >>>> compact memory both directly and by kicking kcompactd in the background. >>>> >>>> It likely should also be per node. >>> Have you benchmarked making the non-huge zero page per-node? >>> >> Not since we disable it :) I will, though. The more concerning issue for >> us, modulo CVE-2017-1000405, is the cpu cost of constantly directly >> compacting memory for allocating the hzp in real time after it has been >> reclaimed. We've observed this happening tens or hundreds of thousands >> of times on some systems. It will be 2MB per node on x86 if the data >> suggests we should make it NUMA aware, I don't think the cost is too high >> to leave it persistently available even under memory pressure if >> use_zero_page is enabled. >> > Measuring access latency to 4GB of memory on Naples I observe ~6.7% > slower access latency intrasocket and ~14% slower intersocket. > > use_zero_page is currently a simple thp flag, meaning it rejects writes > where val != !!val, so perhaps it would be best to overload it with > additional options? I can imagine 0x2 defining persistent allocation so > that the hzp is not freed when the refcount goes to 0 and 0x4 defining if > the hzp should be per node. Implementing persistent allocation fixes our > concern with it, so I'd like to start there. Comments?
Sounds worth trying to me :-) It might be worth making it persistent by default. Keeping 2MB memory unreclaimable sounds not harmful for the use case which prefer to use THP.
| |