Messages in this thread | | | Date | Tue, 22 Sep 2015 12:17:27 +0100 | From | Dave Martin <> | Subject | Re: Question about the sparse memory section size |
| |
On Tue, Sep 22, 2015 at 04:14:21PM +0800, chenfeng wrote: > Hi all, > The sparse memory section size, SECTION_SIZE_BITS, currently is 1GB > for arm64 by default. However, it might generate wasted memmap memory > space for those memory sections less than 1GB. e.g. > > for 512MB memory section, still 14MB(sizeof(struct page) * > PAGES_PER_SECTION) memmap needs to be reserved. The wasted memmap > space could be eliminated by changing memory section size from 1GB to > 512M, but still some questions to be answered, > > 1) why arm64 uses 1GB as default setting? > 2) any risk to change section size from 1GB to 512MB? like, any > impact to performance since memory section number is increased.
For arm64 we have SPARSEMEM_VMEMMAP enabled by default, which enables much of the wasted memmap backing memory to be reclaimed.
Take a look at arch/arm64/mm/init.c:free_unused_memmap().
This should reduce the amount of actual memory wasted on unused parts of memmap. The virtual space stays wasted as you describe, but that's plentiful on 64-bit arches.
You could try sticking some printks in there is you want to see how much of the memmap the code successfully frees.
Cheers ---Dave
| |