Pavel, I assume you are working on the fix. Do you have any progress ? Koki >>-----Original Message----- >>From: Mel Gorman [mailto:mgorman@xxxxxxxxxxxxxxxxxxx] >>Sent: Wednesday, December 06, 2017 5:50 AM >>To: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> >>Cc: Michal Hocko <mhocko@xxxxxxxxxx>; YASUAKI ISHIMATSU >><yasu.isimatu@xxxxxxxxx>; Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; >>Linux Memory Management List <linux-mm@xxxxxxxxx>; linux- >>kernel@xxxxxxxxxxxxxxx; Sanagi, Koki <Koki.Sanagi@xxxxxxxxxxxxxx>; Steve >>Sistare <steven.sistare@xxxxxxxxxx> >>Subject: Re: [PATCH] mm, meminit: Serially initialise deferred memory if >>trace_buf_size is specified >> >>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
