Messages in this thread | | | Date | Thu, 4 Jun 2020 13:22:13 -0400 | From | Daniel Jordan <> | Subject | Re: [PATCH] x86/mm: use max memory block size with unaligned memory end |
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On Thu, Jun 04, 2020 at 09:22:03AM +0200, David Hildenbrand wrote: > On 04.06.20 05:54, Daniel Jordan wrote: > > Some of our servers spend 14 out of the 21 seconds of kernel boot > > initializing memory block sysfs directories and then creating symlinks > > between them and the corresponding nodes. The slowness happens because > > the machines get stuck with the smallest supported memory block size on > > x86 (128M), which results in 16,288 directories to cover the 2T of > > installed RAM, and each of these paths does a linear search of the > > memory blocks for every block id, with atomic ops at each step. > > With 4fb6eabf1037 ("drivers/base/memory.c: cache memory blocks in xarray > to accelerate lookup") merged by Linus' today (strange, I thought this > would be long upstream)
Ah, thanks for pointing this out! It was only posted to LKML so I missed it.
> all linear searches should be gone and at least > the performance observation in this patch no longer applies.
The performance numbers as stated, that's certainly true, but this patch on top still improves kernel boot by 7%. It's a savings of half a second -- I'll take it.
IMHO the root cause of this is really the small block size. Building a cache on top to avoid iterating over tons of small blocks seems like papering over the problem, especially when one of the two affected paths in boot is a cautious check that might be ready to be removed by now[0]:
static int init_memory_block(struct memory_block **memory, unsigned long block_id, unsigned long state) { ... mem = find_memory_block_by_id(block_id); if (mem) { put_device(&mem->dev); return -EEXIST; }
Anyway, I guess I'll redo the changelog and post again.
> The memmap init should nowadays consume most time.
Yeah, but of course it's not as bad as it was now that it's fully parallelized.
[0] https://lore.kernel.org/linux-mm/a8e96df6-dc6d-037f-491c-92182d4ada8d@redhat.com/
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