On Mon, Sep 20, 2021 at 02:53:34AM +0100, Matthew Wilcox wrote: > On Mon, Sep 20, 2021 at 01:09:38AM +0000, Hyeonggon Yoo wrote: > > Hello Matthew, Thanks to give me a comment! I appreciate it. > > > > On Sun, Sep 19, 2021 at 08:17:44PM +0100, Matthew Wilcox wrote: > > > On Sun, Sep 19, 2021 at 04:42:39PM +0000, Hyeonggon Yoo wrote: > > > > It is just simple proof of concept, and not ready for submission yet. > > > > There can be wrong code (like wrong gfp flags, or wrong error handling, > > > > etc) it is just simple proof of concept. I want comment from you. > > > > > > Have you read: > > > > > > https://www.usenix.org/legacy/event/usenix01/full_papers/bonwick/bonwick_html/ > > > The relevant part of that paper is section 3, magazines. We should have > > > low and high water marks for number of objects > > > > I haven't read that before, but after reading it seems not different from > > SLAB's percpu queuing. > > > > > and we should allocate > > > from / free to the slab allocator in batches. Slab has bulk alloc/free > > > APIs already. > > > > > > > There's kmem_cache_alloc_{bulk,free} functions for bulk > > allocation. But it's designed for large number of allocation > > to reduce locking cost, not for percpu lockless allocation. > > What I'm saying is that rather than a linked list of objects, we should > have an array of, say, 15 pointers per CPU (and a count of how many > allocations we have). If we are trying to allocate and have no objects, > call kmem_cache_alloc_bulk() for 8 objects. If we are trying to free > and have 15 objects already, call kmem_cache_free_bulk() for the last > 8 objects and set the number of allocated objects to 7. > > (maybe 8 and 15 are the wrong numbers. this is just an example) > Ah, Okay. it seems better to use array. Using cache for list is unnecessary cost. array is simpler. > > Yeah, we can implement lockless cache using kmem_cache_alloc_{bulk, free} > > but kmem_cache_alloc_{free,bulk} is not enough. > > > > > I'd rather see this be part of the slab allocator than a separate API. > > > > And I disagree on this. for because most of situation, we cannot > > allocate without lock, it is special case for IO polling. > > > > To make it as part of slab allocator, we need to modify existing data > > structure. But making it part of slab allocator will be waste of memory > > because most of them are not using this. > > Oh, it would have to be an option. Maybe as a new slab_flags_t flag. > Or maybe a kmem_cache_alloc_percpu_lockless(). Oh, Now I got what you mean. That is a good improvement! For example, there is a slab_flags_t flag like SLAB_LOCKLESS. and a cache created with SLAB_LOCKLESS flag can allocate using both kmem_cache_alloc, or kmem_cache_alloc_percpu_lockless depending on situation? (I suggest kmem_cache_alloc_lockless is better name) it seems MUCH better. (because it prevents duplicating a cache) I'll send RFC v2 soon. Thank you so much Matthew. If there's misunderstanding from me, please let me know. Thanks, Hyeonggon Yoo
