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From: Joel Fernandes <joel@joelfernandes.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in kfree_rcu()
Date: Fri, 17 Jan 2020 13:57:32 -0500
Message-ID: <20200117185732.GH246464@google.com> (raw)
In-Reply-To: <20200117175217.GA23622@pc636>

On Fri, Jan 17, 2020 at 06:52:17PM +0100, Uladzislau Rezki wrote:
> > > > > But rcuperf uses a single block size, which turns into kfree_bulk() using
> > > > > a single slab, which results in good locality of reference.  So I have to
> > > > 
> > > > You meant a "single cache" category when you say "single slab"? Just to
> > > > mention, the number of slabs (in a single cache) when a large number of
> > > > objects are allocated is more than 1 (not single). With current rcuperf, I
> > > > see 100s of slabs (each slab being one page) in the kmalloc-32 cache. Each
> > > > slab contains around 128 objects of type kfree_rcu (24 byte object aligned to
> > > > 32-byte slab object).
> > > > 
> > > I think that is about using different slab caches to break locality. It
> > > makes sense, IMHO, because usually the system make use of different slabs,
> > > because of different object sizes. From the other hand i guess there are
> > > test cases when only one slab gets used.
> > 
> > I was wondering about "locality". A cache can be split into many slabs. Only
> > the data on a page is local (contiguous). If there are a large number of
> > objects, then it goes to a new slab (on the same cache). At least on the
> > kmalloc slabs, there is only 1 slab per page. So for example, if on
> > kmalloc-32 slab, there are more than 128 objects, then it goes to a different
> > slab / page. So how is there still locality?
> > 
> Hmm.. On a high level:
> 
> one slab cache manages a specific object size, i.e. the slab memory consists of
> contiguous pages(when increased probably not) of memory(4096 bytes or so) divided
> into equal object size. For example when kmalloc() gets called, the appropriate
> cache size(slab that serves only specific size) is selected and an object assigned
> from it is returned.
> 
> But that is theory and i have not deeply analyzed how the SLAB works internally,
> so i can be wrong :)
> 
> You mentioned 128 objects per one slab in the kmalloc-32 slab-cache. But all of
> them follows each other, i mean it is sequential and is like regular array. In

Yes, for these 128 objects it is sequential. But the next 128 could be on
some other page is what I was saying  And we are allocating 10s of 1000s of
objects in this test.  (I believe pages are sequential only per slab and not
for a different slab within same cache).

> that sense freeing can be beneficial because when an access is done to any object
> whole CPU cache-line is fetched(if it was not before), usually it is 64K.

You mean size of the whole L1 cache right? cachelines are in the order of bytes.

> That is what i meant "locality". In order to "break it" i meant to allocate from
> different slabs to see how kfree_slub() behaves in that sense, what is more real
> scenario and workload, i think.

Ok, agreed.
(BTW I do agree your patch is beneficial, just wanted to get the slab
discussion right).

thanks,

 - Joel


  reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-31 12:22 Uladzislau Rezki (Sony)
2020-01-13 19:03 ` Paul E. McKenney
2020-01-14 16:49   ` Joel Fernandes
2020-01-15 13:14     ` Uladzislau Rezki
2020-01-15 22:53       ` Joel Fernandes
2020-01-17 17:52         ` Uladzislau Rezki
2020-01-17 18:57           ` Joel Fernandes [this message]
2020-01-17 21:37             ` Paul E. McKenney
2020-01-17 21:59               ` Joel Fernandes
2020-01-19 13:03                 ` Uladzislau Rezki
2020-01-16  1:14 ` Joel Fernandes
2020-01-16  2:41   ` Paul E. McKenney
2020-01-16 17:27     ` Uladzislau Rezki
2020-01-16 17:44       ` Paul E. McKenney
2020-01-16 17:24   ` Uladzislau Rezki
  -- strict thread matches above, loose matches on Subject: below --
2019-12-20 12:56 Uladzislau Rezki (Sony)
2019-12-21 23:21 ` Joel Fernandes
2019-12-24 18:49   ` Uladzislau Rezki

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