From: Mikulas Patocka <mpatocka@redhat.com>
To: Christopher Lameter <cl@linux.com>
Cc: Mike Snitzer <snitzer@redhat.com>,
Vlastimil Babka <vbabka@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
Pekka Enberg <penberg@kernel.org>,
linux-mm@kvack.org, dm-devel@redhat.com,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE
Date: Thu, 26 Apr 2018 17:09:54 -0400 (EDT) [thread overview]
Message-ID: <alpine.LRH.2.02.1804261508430.26980@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1804261354230.6674@nuc-kabylake>
On Thu, 26 Apr 2018, Christopher Lameter wrote:
> On Wed, 25 Apr 2018, Mikulas Patocka wrote:
>
> > Do you want this? It deletes slab_order and replaces it with the
> > "minimize_waste" logic directly.
>
> Well yes that looks better. Now we need to make it easy to read and less
> complicated. Maybe try to keep as much as possible of the old code
> and also the names of variables to make it easier to review?
>
> > It simplifies the code and it is very similar to the old algorithms, most
> > slab caches have the same order, so it shouldn't cause any regressions.
> >
> > This patch changes order of these slabs:
> > TCPv6: 3 -> 4
> > sighand_cache: 3 -> 4
> > task_struct: 3 -> 4
>
> 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.
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).
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.
> > @@ -3269,35 +3245,35 @@ static inline int calculate_order(unsign
> > max_objects = order_objects(slub_max_order, size, reserved);
> > min_objects = min(min_objects, max_objects);
> >
> > - while (min_objects > 1) {
> > - unsigned int fraction;
> > + /* Get the minimum acceptable order for one object */
> > + order = get_order(size + reserved);
> > +
> > + for (test_order = order + 1; test_order < MAX_ORDER; test_order++) {
> > + unsigned order_obj = order_objects(order, size, reserved);
> > + unsigned test_order_obj = order_objects(test_order, size, reserved);
> > +
> > + /* If there are too many objects, stop searching */
> > + if (test_order_obj > MAX_OBJS_PER_PAGE)
> > + break;
> >
> > - fraction = 16;
> > - while (fraction >= 4) {
> > - order = slab_order(size, min_objects,
> > - slub_max_order, fraction, reserved);
> > - if (order <= slub_max_order)
> > - return order;
> > - fraction /= 2;
> > - }
> > - min_objects--;
> > + /* Always increase up to slub_min_order */
> > + if (test_order <= slub_min_order)
> > + order = test_order;
>
> Well that is a significant change. In our current scheme the order
> boundart wins.
I think it's not a change. The existing function slab_order() starts with
min_order (unless it overshoots MAX_OBJS_PER_PAGE) and then goes upwards.
My code does the same - my code tests for MAX_OBJS_PER_PAGE (and bails out
if we would overshoot it) and increases the order until it reaches
slub_min_order (and then increases it even more if it satisfies the other
conditions).
If you believe that it behaves differently, please describe the situation
in detail.
> > +
> > + /* If we are below min_objects and slub_max_order, increase order */
> > + if (order_obj < min_objects && test_order <= slub_max_order)
> > + order = test_order;
> > +
> > + /* 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
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).
> > + test_order_obj > order_obj << (test_order - order))
>
> Add more () to make the condition better readable.
>
> > + order = test_order;
>
> Can we just call test_order order and avoid using the long variable names
> here? Variable names in functions are typically short.
You need two variables - "order" and "test_order".
"order" is the best order found so far and "test_order" is the order that
we are now testing. If "test_order" wastes less space than "order", we
assign order = test_order.
Mikulas
next prev parent reply other threads:[~2018-04-26 21:10 UTC|newest]
Thread overview: 109+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-20 17:25 [PATCH] slab: introduce the flag SLAB_MINIMIZE_WASTE Mikulas Patocka
2018-03-20 17:35 ` Matthew Wilcox
2018-03-20 17:35 ` Matthew Wilcox
2018-03-20 17:54 ` Christopher Lameter
2018-03-20 17:54 ` Christopher Lameter
2018-03-20 19:22 ` Mikulas Patocka
2018-03-20 19:22 ` Mikulas Patocka
2018-03-20 20:42 ` Christopher Lameter
2018-03-20 20:42 ` Christopher Lameter
2018-03-20 22:02 ` Mikulas Patocka
2018-03-20 22:02 ` Mikulas Patocka
2018-03-21 15:35 ` Christopher Lameter
2018-03-21 15:35 ` Christopher Lameter
2018-03-21 16:25 ` Mikulas Patocka
2018-03-21 16:25 ` Mikulas Patocka
2018-03-21 17:10 ` Matthew Wilcox
2018-03-21 17:10 ` Matthew Wilcox
2018-03-21 17:30 ` Christopher Lameter
2018-03-21 17:30 ` Christopher Lameter
2018-03-21 17:39 ` Christopher Lameter
2018-03-21 17:39 ` Christopher Lameter
2018-03-21 17:49 ` Matthew Wilcox
2018-03-21 17:49 ` Matthew Wilcox
2018-03-21 18:01 ` Christopher Lameter
2018-03-21 18:01 ` Christopher Lameter
2018-03-21 18:23 ` Mikulas Patocka
2018-03-21 18:23 ` Mikulas Patocka
2018-03-21 18:40 ` Christopher Lameter
2018-03-21 18:40 ` Christopher Lameter
2018-03-21 18:55 ` Mikulas Patocka
2018-03-21 18:55 ` Mikulas Patocka
2018-03-21 18:55 ` Matthew Wilcox
2018-03-21 18:55 ` Matthew Wilcox
2018-03-21 18:58 ` Christopher Lameter
2018-03-21 18:58 ` Christopher Lameter
2018-03-21 19:25 ` Mikulas Patocka
2018-03-21 19:25 ` Mikulas Patocka
2018-03-21 18:36 ` Mikulas Patocka
2018-03-21 18:36 ` Mikulas Patocka
2018-03-21 18:57 ` Christopher Lameter
2018-03-21 18:57 ` Christopher Lameter
2018-03-21 19:19 ` Mikulas Patocka
2018-03-21 19:19 ` Mikulas Patocka
2018-03-21 20:09 ` Christopher Lameter
2018-03-21 20:09 ` Christopher Lameter
2018-03-21 20:37 ` Mikulas Patocka
2018-03-21 20:37 ` Mikulas Patocka
2018-03-23 15:10 ` Christopher Lameter
2018-03-23 15:10 ` Christopher Lameter
2018-03-23 15:31 ` Mikulas Patocka
2018-03-23 15:31 ` Mikulas Patocka
2018-03-23 15:48 ` Christopher Lameter
2018-03-23 15:48 ` Christopher Lameter
2018-04-13 9:22 ` Vlastimil Babka
2018-04-13 9:22 ` Vlastimil Babka
2018-04-13 15:10 ` Mike Snitzer
2018-04-13 15:10 ` Mike Snitzer
2018-04-16 12:38 ` Vlastimil Babka
2018-04-16 12:38 ` Vlastimil Babka
2018-04-16 14:27 ` Mike Snitzer
2018-04-16 14:27 ` Mike Snitzer
2018-04-16 14:37 ` Mikulas Patocka
2018-04-16 14:37 ` Mikulas Patocka
2018-04-16 14:46 ` Mike Snitzer
2018-04-16 14:46 ` Mike Snitzer
2018-04-16 14:57 ` Mikulas Patocka
2018-04-16 14:57 ` Mikulas Patocka
2018-04-16 15:18 ` Christopher Lameter
2018-04-16 15:18 ` Christopher Lameter
2018-04-16 15:25 ` Mikulas Patocka
2018-04-16 15:25 ` Mikulas Patocka
2018-04-16 15:45 ` Christopher Lameter
2018-04-16 15:45 ` Christopher Lameter
2018-04-16 19:36 ` Mikulas Patocka
2018-04-16 19:36 ` Mikulas Patocka
2018-04-16 19:53 ` Vlastimil Babka
2018-04-16 21:01 ` Mikulas Patocka
2018-04-17 14:40 ` Christopher Lameter
2018-04-17 18:53 ` Mikulas Patocka
2018-04-17 18:53 ` Mikulas Patocka
2018-04-17 21:42 ` Christopher Lameter
2018-04-17 14:49 ` Christopher Lameter
2018-04-17 14:49 ` Christopher Lameter
2018-04-17 14:47 ` Christopher Lameter
2018-04-17 14:47 ` Christopher Lameter
2018-04-16 19:32 ` [PATCH RESEND] " Mikulas Patocka
2018-04-17 14:45 ` Christopher Lameter
2018-04-17 16:16 ` Vlastimil Babka
2018-04-17 16:38 ` Christopher Lameter
2018-04-17 19:09 ` Mikulas Patocka
2018-04-17 17:26 ` Mikulas Patocka
2018-04-17 19:13 ` Vlastimil Babka
2018-04-17 19:06 ` Mikulas Patocka
2018-04-17 19:06 ` Mikulas Patocka
2018-04-18 14:55 ` Christopher Lameter
2018-04-25 21:04 ` Mikulas Patocka
2018-04-25 23:24 ` Mikulas Patocka
2018-04-26 19:01 ` Christopher Lameter
2018-04-26 21:09 ` Mikulas Patocka [this message]
2018-04-27 16:41 ` Christopher Lameter
2018-04-27 19:19 ` Mikulas Patocka
2018-06-13 17:01 ` Mikulas Patocka
2018-06-13 18:16 ` Christoph Hellwig
2018-06-13 18:53 ` Mikulas Patocka
2018-04-26 18:51 ` Christopher Lameter
2018-04-16 19:38 ` Vlastimil Babka
2018-04-16 19:38 ` Vlastimil Babka
2018-04-16 21:04 ` Mikulas Patocka
2018-04-16 21:04 ` Mikulas Patocka
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