On Thu 04-03-21 16:14:14, Feng Tang wrote: > On Wed, Mar 03, 2021 at 09:22:50AM -0800, Ben Widawsky wrote: > > On 21-03-03 18:14:30, Michal Hocko wrote: > > > On Wed 03-03-21 08:31:41, Ben Widawsky wrote: > > > > On 21-03-03 14:59:35, Michal Hocko wrote: > > > > > On Wed 03-03-21 21:46:44, Feng Tang wrote: > > > > > > On Wed, Mar 03, 2021 at 09:18:32PM +0800, Tang, Feng wrote: > > > > > > > On Wed, Mar 03, 2021 at 01:32:11PM +0100, Michal Hocko wrote: > > > > > > > > On Wed 03-03-21 20:18:33, Feng Tang wrote: > > > > > [...] > > > > > > > > > One thing I tried which can fix the slowness is: > > > > > > > > > > > > > > > > > > + gfp_mask &= ~(__GFP_DIRECT_RECLAIM | __GFP_KSWAPD_RECLAIM); > > > > > > > > > > > > > > > > > > which explicitly clears the 2 kinds of reclaim. And I thought it's too > > > > > > > > > hacky and didn't mention it in the commit log. > > > > > > > > > > > > > > > > Clearing __GFP_DIRECT_RECLAIM would be the right way to achieve > > > > > > > > GFP_NOWAIT semantic. Why would you want to exclude kswapd as well? > > > > > > > > > > > > > > When I tried gfp_mask &= ~__GFP_DIRECT_RECLAIM, the slowness couldn't > > > > > > > be fixed. > > > > > > > > > > > > I just double checked by rerun the test, 'gfp_mask &= ~__GFP_DIRECT_RECLAIM' > > > > > > can also accelerate the allocation much! though is still a little slower than > > > > > > this patch. Seems I've messed some of the tries, and sorry for the confusion! > > > > > > > > > > > > Could this be used as the solution? or the adding another fallback_nodemask way? > > > > > > but the latter will change the current API quite a bit. > > > > > > > > > > I haven't got to the whole series yet. The real question is whether the > > > > > first attempt to enforce the preferred mask is a general win. I would > > > > > argue that it resembles the existing single node preferred memory policy > > > > > because that one doesn't push heavily on the preferred node either. So > > > > > dropping just the direct reclaim mode makes some sense to me. > > > > > > > > > > IIRC this is something I was recommending in an early proposal of the > > > > > feature. > > > > > > > > My assumption [FWIW] is that the usecases we've outlined for multi-preferred > > > > would want more heavy pushing on the preference mask. However, maybe the uapi > > > > could dictate how hard to try/not try. > > > > > > What does that mean and what is the expectation from the kernel to be > > > more or less cast in stone? > > > > > > > (I'm not positive I've understood your question, so correct me if I > > misunderstood) > > > > I'm not sure there is a stone-cast way to define it nor should we. At the very > > least though, something in uapi that has a general mapping to GFP flags > > (specifically around reclaim) for the first round of allocation could make > > sense. > > > > In my head there are 3 levels of request possible for multiple nodes: > > 1. BIND: Those nodes or die. > > 2. Preferred hard: Those nodes and I'm willing to wait. Fallback if impossible. > > 3. Preferred soft: Those nodes but I don't want to wait. > > > > Current UAPI in the series doesn't define a distinction between 2, and 3. As I > > understand the change, Feng is defining the behavior to be #3, which makes #2 > > not an option. I sort of punted on defining it entirely, in the beginning. > > As discussed earlier in the thread, one less hacky solution is to clear > __GFP_DIRECT_RECLAIM bit so that it won't go into direct reclaim, but still > wakeup the kswapd of target nodes and retry, which sits now between 'Preferred hard' > and 'Preferred soft' :) Yes that is what I've had in mind when talking about a lightweight attempt. > For current MPOL_PREFERRED, its semantic is also 'Preferred hard', that it Did you mean to say prefer soft? Because the direct reclaim is attempted only when node reclaim is enabled. > will check free memory of other nodes before entering slowpath waiting. Yes, hence "soft" semantic. -- Michal Hocko SUSE Labs
