[dm-devel] [PATCH] slab: introduce the flag SLAB_MINIMIZE_WASTE

[Mikulas Patocka]

On Tue, 20 Mar 2018, Christopher Lameter wrote:

> On Tue, 20 Mar 2018, Matthew Wilcox wrote:
> 
> > On Tue, Mar 20, 2018 at 01:25:09PM -0400, Mikulas Patocka wrote:
> > > The reason why we need this is that we are going to merge code that does
> > > block device deduplication (it was developed separatedly and sold as a
> > > commercial product), and the code uses block sizes that are not a power of
> > > two (block sizes 192K, 448K, 640K, 832K are used in the wild). The slab
> > > allocator rounds up the allocation to the nearest power of two, but that
> > > wastes a lot of memory. Performance of the solution depends on efficient
> > > memory usage, so we should minimize wasted as much as possible.
> >
> > The SLUB allocator also falls back to using the page (buddy) allocator
> > for allocations above 8kB, so this patch is going to have no effect on
> > slub.  You'd be better off using alloc_pages_exact() for this kind of
> > size, or managing your own pool of pages by using something like five
> > 192k blocks in a 1MB allocation.
> 
> The fallback is only effective for kmalloc caches. Manually created caches
> do not follow this rule.

Yes - the dm-bufio layer uses manually created caches.

> Note that you can already control the page orders for allocation and
> the objects per slab using
> 
> 	slub_min_order
> 	slub_max_order
> 	slub_min_objects
> 
> This is documented in linux/Documentation/vm/slub.txt
> 
> Maybe do the same thing for SLAB?

Yes, but I need to change it for a specific cache, not for all caches.

When the order is greater than 3 (PAGE_ALLOC_COSTLY_ORDER), the allocation 
becomes unreliable, thus it is a bad idea to increase slub_max_order 
system-wide.

Another problem with slub_max_order is that it would pad all caches to 
slub_max_order, even those that already have a power-of-two size (in that 
case, the padding is counterproductive).

BTW. the function "order_store" in mm/slub.c modifies the structure 
kmem_cache without taking any locks - is it a bug?

Mikulas