On Thu, 26 Apr 2018, Mikulas Patocka wrote: > > Hmmm... order 4 for these caches may cause some concern. These should stay > > under costly order I think. Otherwise allocations are no longer > > guaranteed. > > You said that slub has fallback to smaller order allocations. Yes it does... > The whole purpose of this "minimize waste" approach is to use higher-order > allocations to use memory more efficiently, so it is just doing its job. > (for these 3 caches, order-4 really wastes less memory than order-3 - on > my system TCPv6 and sighand_cache have size 2112, task_struct 2752). Hmmm... Ok if the others are fine with this as well. I got some pushback there in the past. > We could improve the fallback code, so that if order-4 allocation fails, > it tries order-3 allocation, and then falls back to order-0. But I think > that these failures are rare enough that it is not a problem. I also think that would be too many fallbacks. > > > + /* Increase order even more, but only if it reduces waste */ > > > + if (test_order_obj <= 32 && > > > > Where does the 32 come from? > > It is to avoid extremely high order for extremely small slabs. > > For example, see kmalloc-96. > 10922 96-byte objects would fit into 1MiB > 21845 96-byte objects would fit into 2MiB That is the result of considering absolute byte wastage.. > The algorithm would recognize this one more object that fits into 2MiB > slab as "waste reduction" and increase the order to 2MiB - and we don't > want this. > > So, the general reasoning is - if we have 32 objects in a slab, then it is > already considered that wasted space is reasonably low and we don't want > to increase the order more. > > Currently, kmalloc-96 uses order-0 - that is reasonable (we already have > 42 objects in 4k page, so we don't need to use higher order, even if it > wastes one-less object). The old code uses the concept of a "fraction" to calculate overhead. The code here uses absolute counts of bytes. Fraction looks better to me.
