On Mon, May 25, 2015 at 07:01:33PM +0900, Jungseok Lee wrote: > On May 25, 2015, at 2:49 AM, Arnd Bergmann wrote: > > On Monday 25 May 2015 01:02:20 Jungseok Lee wrote: > >> Fork-routine sometimes fails to get a physically contiguous region for > >> thread_info on 4KB page system although free memory is enough. That is, > >> a physically contiguous region, which is currently 16KB, is not available > >> since system memory is fragmented. > >> > >> This patch tries to solve the problem as allocating thread_info memory > >> from vmalloc space, not 1:1 mapping one. The downside is one additional > >> page allocation in case of vmalloc. However, vmalloc space is large enough, > >> around 240GB, under a combination of 39-bit VA and 4KB page. Thus, it is > >> not a big tradeoff for fork-routine service. > > > > vmalloc has a rather large runtime cost. I'd argue that failing to allocate > > thread_info structures means something has gone very wrong. > > That is why the feature is marked "N" by default. > I focused on fork-routine stability rather than performance. If VM has trouble with order-2 allocation, your system would be trouble soon although fork at the moment manages to be successful because such small high-order(ex, order <= PAGE_ALLOC_COSTLY_ORDER) allocation is common in the kernel so VM should handle it smoothly. If VM didn't, it means we should fix VM itself, not a specific allocation site. Fork is one of victim by that. > > Could you give me an idea how to evaluate performance degradation? > Running some benchmarks would be helpful, but I would like to try to > gather data based on meaningful methodology. > > > Can you describe the scenario that leads to fragmentation this bad? > > Android, but I could not describe an exact reproduction procedure step > by step since it's behaved and reproduced randomly. As reading the following > thread from mm mailing list, a similar symptom is observed on other systems. > > https://lkml.org/lkml/2015/4/28/59 > > Although I do not know the details of a system mentioned in the thread, > even order-2 page allocation is not smoothly operated due to fragmentation on > low memory system. What Joonsoo have tackle is generic fragmentation problem, not *a* fork fail, which is more right approach to handle small high-order allocation problem. > > I think the point is *low memory system*. 64-bit kernel is usually a feasible > option when system memory is enough, but 64-bit kernel and low memory system > combo is not unusual in case of ARM64. > > > Could the stack size be reduced to 8KB perhaps? > > I guess probably not. > > A commit, 845ad05e, says that 8KB is not enough to cover SpecWeb benchmark. > The stack size is 16KB on x86_64. I am not sure whether all applications, > which work fine on x86_64 machine, run very well on ARM64 with 8KB stack size. > > Best Regards > Jungseok Lee > -- > 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> -- Kind regards, Minchan Kim -- 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>