Messages in this thread | | | Date | Wed, 4 Nov 2015 14:54:46 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] arm64: Increase the max granular size |
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On Wed, Nov 04, 2015 at 07:53:50AM -0600, Christoph Lameter wrote: > On Wed, 4 Nov 2015, Catalin Marinas wrote: > > > The simplest option would be to make sure that off slab isn't allowed > > for caches of KMALLOC_MIN_SIZE or smaller, with the drawback that not > > only "kmalloc-128" but any other such caches will be on slab. > > The reason for an off slab configuration is denser object packing. > > > I think a better option would be to first check that there is a > > kmalloc_caches[] entry for freelist_size before deciding to go off-slab. > > Hmmm.. Yes seems to be an option. > > Maybe we simply revert commit 8fc9cf420b36 instead?
I'm fine with this. Also note that the arm64 commit changing L1_CACHE_BYTES to 128 hasn't been pushed yet (it's queued for 4.4).
> That does not seem to make too much sense to me and the goal of the > commit cannot be accomplished on ARM. Your patch essentially reverts > the effect anyways.
In theory it only reverts the effect for the first kmalloc_cache ("kmalloc-128" in the arm64 case). Any other bigger cache which would not be mergeable with an existing one still has the potential of off-slab management.
> Smaller slabs really do not need off slab management anyways since they > will only loose a few objects per slab page.
IIUC, starting with 128 slab size for a 4KB page, you have 32 objects per page. The freelist takes 32 bytes (or 31), therefore you waste a single slab object. However, only 1/4 of it is used for freelist and the waste gets bigger with 256 slab size, hence the original commit.
BTW, assuming L1_CACHE_BYTES is 512 (I don't ever see this happening but just in theory), we potentially have the same issue. What would save us is that INDEX_NODE would match the first "kmalloc-512" cache, so we have it pre-populated.
-- Catalin
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