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Date: Mon, 20 Sep 2021 11:55:36 +0000 From: Hyeonggon Yoo <42.hyeyoo@...il.com> To: Vlastimil Babka <vbabka@...e.cz> Cc: Matthew Wilcox <willy@...radead.org>, Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk> Subject: Re: [RFC PATCH] Introducing lockless cache built on top of slab allocator On Mon, Sep 20, 2021 at 11:07:36AM +0200, Vlastimil Babka wrote: > On 9/20/21 03:53, 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. > >> 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(). > > I've recently found out that similar attempts (introduce queueing to SLUB) > have been done around 2010. See e.g. [1] but there will be other threads to > search at lore too. Haven't checked yet while it wasn't ultimately merged, > I guess Christoph and David could remember (this was before my time). There was attempt on SLUB with queueing as you said. I searched a bit and found [2] and [3]. - SLUB with queueing (V2) beats SLAB netperf TCP_RR, 2010-07 [2] https://lore.kernel.org/lkml/alpine.DEB.2.00.1007121010420.14328@router.home/T/#m5a31c7caa28b93a00de3af6d547b79273449f5ba - The Unified slab allocator (V4), 2010-10 [3] https://linux-mm.kvack.narkive.com/e595iCuz/unifiedv4-00-16-the-unified-slab-allocator-v4#post47 Looking at [3], There was still some regression comparing "SLUB with queueing" with SLAB. And I couldn't find patch series after [3] yet. I'll add link if I find. > I guess making it opt-in only for caches where performance improvement was > measured would make it easier to add, as for some caches it would mean no > improvement, but increased memory usage. But of course it makes the API more > harder to use. Do you mean "lockless cache" it should be separate from slab because some caches doesn't benefit at all? > I'd be careful about the name "lockless", as that's ambiguous. Is it "mostly > lockless" therefore fast, but if the cache is empty, it will still take > locks as part of refill? It is actually "mostly lockless" so it is ambiguous. Can you suggest a name? like try_lockless or anything? > Or is it lockless always, therefore useful in > contexts that can take no locks, but then the caller has to have fallbacks > in case the cache is empty and nothing is allocated? > > [1] https://lore.kernel.org/linux-mm/20100804024531.914852850@linux.com/T/#u
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