On Thu 29-08-19 10:03:21, Yang Shi wrote: > On Wed, Aug 28, 2019 at 9:02 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Wed 28-08-19 17:46:59, Kirill A. Shutemov wrote: > > > On Wed, Aug 28, 2019 at 02:12:53PM +0000, Michal Hocko wrote: > > > > On Wed 28-08-19 17:03:29, Kirill A. Shutemov wrote: > > > > > On Wed, Aug 28, 2019 at 09:57:08AM +0200, Michal Hocko wrote: > > > > > > On Tue 27-08-19 10:06:20, Yang Shi wrote: > > > > > > > > > > > > > > > > > > > > > On 8/27/19 5:59 AM, Kirill A. Shutemov wrote: > > > > > > > > On Tue, Aug 27, 2019 at 03:17:39PM +0300, Kirill A. Shutemov wrote: > > > > > > > > > On Tue, Aug 27, 2019 at 02:09:23PM +0200, Michal Hocko wrote: > > > > > > > > > > On Tue 27-08-19 14:01:56, Vlastimil Babka wrote: > > > > > > > > > > > On 8/27/19 1:02 PM, Kirill A. Shutemov wrote: > > > > > > > > > > > > On Tue, Aug 27, 2019 at 08:01:39AM +0200, Michal Hocko wrote: > > > > > > > > > > > > > On Mon 26-08-19 16:15:38, Kirill A. Shutemov wrote: > > > > > > > > > > > > > > Unmapped completely pages will be freed with current code. Deferred split > > > > > > > > > > > > > > only applies to partly mapped THPs: at least on 4k of the THP is still > > > > > > > > > > > > > > mapped somewhere. > > > > > > > > > > > > > Hmm, I am probably misreading the code but at least current Linus' tree > > > > > > > > > > > > > reads page_remove_rmap -> [page_remove_anon_compound_rmap ->\ deferred_split_huge_page even > > > > > > > > > > > > > for fully mapped THP. > > > > > > > > > > > > Well, you read correctly, but it was not intended. I screwed it up at some > > > > > > > > > > > > point. > > > > > > > > > > > > > > > > > > > > > > > > See the patch below. It should make it work as intened. > > > > > > > > > > > > > > > > > > > > > > > > It's not bug as such, but inefficientcy. We add page to the queue where > > > > > > > > > > > > it's not needed. > > > > > > > > > > > But that adding to queue doesn't affect whether the page will be freed > > > > > > > > > > > immediately if there are no more partial mappings, right? I don't see > > > > > > > > > > > deferred_split_huge_page() pinning the page. > > > > > > > > > > > So your patch wouldn't make THPs freed immediately in cases where they > > > > > > > > > > > haven't been freed before immediately, it just fixes a minor > > > > > > > > > > > inefficiency with queue manipulation? > > > > > > > > > > Ohh, right. I can see that in free_transhuge_page now. So fully mapped > > > > > > > > > > THPs really do not matter and what I have considered an odd case is > > > > > > > > > > really happening more often. > > > > > > > > > > > > > > > > > > > > That being said this will not help at all for what Yang Shi is seeing > > > > > > > > > > and we need a more proactive deferred splitting as I've mentioned > > > > > > > > > > earlier. > > > > > > > > > It was not intended to fix the issue. It's fix for current logic. I'm > > > > > > > > > playing with the work approach now. > > > > > > > > Below is what I've come up with. It appears to be functional. > > > > > > > > > > > > > > > > Any comments? > > > > > > > > > > > > > > Thanks, Kirill and Michal. Doing split more proactive is definitely a choice > > > > > > > to eliminate huge accumulated deferred split THPs, I did think about this > > > > > > > approach before I came up with memcg aware approach. But, I thought this > > > > > > > approach has some problems: > > > > > > > > > > > > > > First of all, we can't prove if this is a universal win for the most > > > > > > > workloads or not. For some workloads (as I mentioned about our usecase), we > > > > > > > do see a lot THPs accumulated for a while, but they are very short-lived for > > > > > > > other workloads, i.e. kernel build. > > > > > > > > > > > > > > Secondly, it may be not fair for some workloads which don't generate too > > > > > > > many deferred split THPs or those THPs are short-lived. Actually, the cpu > > > > > > > time is abused by the excessive deferred split THPs generators, isn't it? > > > > > > > > > > > > Yes this is indeed true. Do we have any idea on how much time that > > > > > > actually is? > > > > > > > > > > For uncontented case, splitting 1G worth of pages (2MiB x 512) takes a bit > > > > > more than 50 ms in my setup. But it's best-case scenario: pages not shared > > > > > across multiple processes, no contention on ptl, page lock, etc. > > > > > > > > Any idea about a bad case? > > > > > > Not really. > > > > > > How bad you want it to get? How many processes share the page? Access > > > pattern? Locking situation? > > > > Let's say how hard a regular user can make this? > > > > > Worst case scenarion: no progress on splitting due to pins or locking > > > conflicts (trylock failure). > > > > > > > > > > With memcg awareness, the deferred split THPs actually are isolated and > > > > > > > capped by memcg. The long-lived deferred split THPs can't be accumulated too > > > > > > > many due to the limit of memcg. And, cpu time spent in splitting them would > > > > > > > just account to the memcgs who generate that many deferred split THPs, who > > > > > > > generate them who pay for it. This sounds more fair and we could achieve > > > > > > > much better isolation. > > > > > > > > > > > > On the other hand, deferring the split and free up a non trivial amount > > > > > > of memory is a problem I consider quite serious because it affects not > > > > > > only the memcg workload which has to do the reclaim but also other > > > > > > consumers of memory beucase large memory blocks could be used for higher > > > > > > order allocations. > > > > > > > > > > Maybe instead of drive the split from number of pages on queue we can take > > > > > a hint from compaction that is struggles to get high order pages? > > > > > > > > This is still unbounded in time. > > > > > > I'm not sure we should focus on time. > > > > > > We need to make sure that we don't overal system health worse. Who cares > > > if we have pages on deferred split list as long as we don't have other > > > user for the memory? > > > > We do care for all those users which do not want to get stalled when > > requesting that memory. And you cannot really predict that, right? So > > the sooner the better. Modulo time wasted for the pointless splitting of > > course. I am afraid defining the best timing here is going to be hard > > but let's focus on workloads that are known to generate partial THPs and > > see how that behaves. > > I'm supposed we are just concerned by the global memory pressure > incurred by the excessive deferred split THPs. As long as no other > users for that memory we don't have to waste time to care about it. > So, I'm wondering why not we do harder in kswapd? kswapd is already late. There shouldn't be any need for the reclaim as long as there is a lot of memory that can be directly freed. > Currently, deferred split THPs get shrunk like slab. The number of > objects scanned is determined by some factors, i.e. scan priority, > shrinker->seeks, etc, to avoid over reclaim for filesystem caches to > avoid extra I/O. But, we don't have to worry about over reclaim for > deferred split THPs, right? We definitely could shrink them more > aggressively in kswapd context. This is certainly possible. I am just wondering why should we cram this into the reclaim when we have a reasonable trigger to do that. > For example, we could simply set shrinker->seeks to 0, now it is > DEFAULT_SEEKS. > > And, we also could consider boost water mark to wake up kswapd earlier > once we see excessive deferred split THPs accumulated. This has other side effect, right? -- Michal Hocko SUSE Labs
