On Wed, Nov 29, 2017 at 10:41:59PM -0500, Pavel Tatashin wrote: > Hi Mel, > > Thank you very much for your feedback, my replies below: > > > A lack of involvement from admins is indeed desirable. For example, > > while I might concede on using a disable-everything-switch, I would not > > be happy to introduce a switch that specified how much memory per node > > to initialise. > > > > For the forth approach, I really would be only thinking of a blunt > > "initialise everything instead of going OOM". I was wary of making things > > too complicated and I worried about some side-effects I'll cover later. > > I see, I misunderstood your suggestion. Switching to serial > initialization when OOM works, however, boot time becomes > unpredictable, with some configurations boot is fast with others it is > slow. All of that depends on whether predictions in > reset_deferred_meminit() were good or not which is not easy to debug > for users. Also, overtime predictions in reset_deferred_meminit() can > become very off, and I do not think that we want to continuously > adjust this function. > You could increase the probabilty of a report by doing a WARN_ON_ONCE if the serialised meminit is used. > >> With this approach we could always init a very small amount of struct > >> pages, and allow the rest to be initialized on demand as boot requires > >> until deferred struct pages are initialized. Since, having deferred > >> pages feature assumes that the machine is large, there is no drawback > >> of having some extra byte of dead code, especially that all the checks > >> can be permanently switched of via static branches once deferred init > >> is complete. > >> > > > > This is where I fear there may be dragons. If we minimse the number of > > struct pages and initialise serially as necessary, there is a danger that > > we'll allocate remote memory in cases where local memory would have done > > because a remote node had enough memory. > > True, but is not what we have now has the same issue as well? If one > node is gets out of memory we start using memory from another node, > before deferred pages are initialized? > It's possible but I'm not aware of it happening currently. > To offset that risk, it would be > > necessary at boot-time to force allocations from local node where possible > > and initialise more memory as necessary. That starts getting complicated > > because we'd need to adjust gfp-flags in the fast path with init-and-retry > > logic in the slow path and that could be a constant penalty. We could offset > > that in the fast path by using static branches > > I will try to implement this, and see how complicated the patch will > be, if it gets too complicated for the problem I am trying to solve we > can return to one of your suggestions. > > I was thinking to do something like this: > > Start with every small amount of initialized pages in every node. > If allocation fails, initialize enough struct pages to cover this > particular allocation with struct pages rounded up to section size but > in every single node. > Ok, just make sure it's all in the slow paths of the allocator when the alternative is to fail the allocation. > > but it's getting more and > > more complex for what is a minor optimisation -- shorter boot times on > > large machines where userspace itself could take a *long* time to get up > > and running (think database reading in 1TB of data from disk as it warms up). > > On M6-32 with 32T [1] of memory it saves over 4 minutes of boot time, > and this is on SPARC with 8K pages, on x86 it would be around of 8 > minutes because of twice as many pages. This feature improves > availability for larger machines quite a bit. Overtime, systems are > growing, so I expect this feature to become a default configuration in > the next several years on server configs. > Ok, when developing the series originally, I had no machine even close to 32T of memory. -- Mel Gorman SUSE Labs -- 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>
