linux-kernel - Re: [RFC PATCH] Introducing lockless cache built on top of slab allocator

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Message-ID: <20210920010938.GA3108@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
Date:   Mon, 20 Sep 2021 01:09:38 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, 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

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.

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.