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From: Christopher Lameter <cl@linux.com>
To: Mikulas Patocka <mpatocka@redhat.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: Fri, 27 Apr 2018 11:41:48 -0500 (CDT)	[thread overview]
Message-ID: <alpine.DEB.2.20.1804271136390.11686@nuc-kabylake> (raw)
In-Reply-To: <alpine.LRH.2.02.1804261508430.26980@file01.intranet.prod.int.rdu2.redhat.com>

On Thu, 26 Apr 2018, Mikulas Patocka wrote:

> > 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.

Yes it does...

> 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).

Hmmm... Ok if the others are fine with this as well. I got some pushback
there in the past.

> 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.

I also think that would be too many fallbacks.

> > > +		/* 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

That is the result of considering absolute byte wastage..

> 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).


The old code uses the concept of a "fraction" to calculate overhead. The
code here uses absolute counts of bytes. Fraction looks better to me.

  reply	other threads:[~2018-04-27 16:41 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <alpine.LRH.2.02.1803201740280.21066@file01.intranet.prod.int.rdu2.redhat.com>
     [not found] ` <alpine.DEB.2.20.1803211024220.2175@nuc-kabylake>
     [not found]   ` <alpine.LRH.2.02.1803211153320.16017@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]     ` <alpine.DEB.2.20.1803211226350.3174@nuc-kabylake>
     [not found]       ` <alpine.LRH.2.02.1803211425330.26409@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]         ` <20c58a03-90a8-7e75-5fc7-856facfb6c8a@suse.cz>
     [not found]           ` <20180413151019.GA5660@redhat.com>
     [not found]             ` <ee8807ff-d650-0064-70bf-e1d77fa61f5c@suse.cz>
     [not found]               ` <20180416142703.GA22422@redhat.com>
     [not found]                 ` <alpine.LRH.2.02.1804161031300.24222@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]                   ` <20180416144638.GA22484@redhat.com>
2018-04-16 19:32                     ` 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
2018-04-18 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
2018-04-27 16:41                                     ` Christopher Lameter [this message]
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
     [not found]                     ` <alpine.LRH.2.02.1804161054410.17807@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]                       ` <alpine.DEB.2.20.1804161018030.9397@nuc-kabylake>
     [not found]                         ` <alpine.LRH.2.02.1804161123400.17807@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]                           ` <alpine.DEB.2.20.1804161043430.9622@nuc-kabylake>
     [not found]                             ` <alpine.LRH.2.02.1804161532480.19492@file01.intranet.prod.int.rdu2.redhat.com>
     [not found]                               ` <b0e6ccf6-06ce-e50b-840e-c8d3072382fd@suse.cz>
2018-04-16 21:01                                 ` Mikulas Patocka
2018-04-17 14:40                                   ` Christopher Lameter
2018-04-17 18:53                                     ` Mikulas Patocka
2018-04-17 21:42                                       ` Christopher Lameter

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