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. 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. 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