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From: Mikulas Patocka <mpatocka@redhat.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Christopher Lameter <cl@linux.com>,
	Mike Snitzer <snitzer@redhat.com>,
	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: Tue, 17 Apr 2018 13:26:51 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.1804171318010.5023@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <f8f736fe-9e0e-acd2-8040-f4f25ea5a7a2@suse.cz>



On Tue, 17 Apr 2018, Vlastimil Babka wrote:

> On 04/17/2018 04:45 PM, Christopher Lameter wrote:
> > On Mon, 16 Apr 2018, Mikulas Patocka wrote:
> > 
> >> This patch introduces a flag SLAB_MINIMIZE_WASTE for slab and slub. This
> >> flag causes allocation of larger slab caches in order to minimize wasted
> >> space.
> >>
> >> This is needed because we want to use dm-bufio for deduplication index and
> >> there are existing installations with non-power-of-two block sizes (such
> >> as 640KB). The performance of the whole solution depends on efficient
> >> memory use, so we must waste as little memory as possible.
> > 
> > Hmmm. Can we come up with a generic solution instead?
> 
> Yes please.
> 
> > This may mean relaxing the enforcement of the allocation max order a bit
> > so that we can get dense allocation through higher order allocs.
> > 
> > But then higher order allocs are generally seen as problematic.
> 
> I think in this case they are better than wasting/fragmenting 384kB for
> 640kB object.

Wasting 37% of memory is still better than the kernel randomly returning 
-ENOMEM when higher-order allocation fails.

> > That
> > means that callers need to be able to tolerate failures.
> 
> Is it any different from now? I suppose there would still be
> smallest-order fallback involved in sl*b itself? And if your allocation
> is so large it can fail even with the fallback (i.e. >= costly order),
> you need to tolerate failures anyway?
> 
> One corner case I see is if there is anyone who would rather use their
> own fallback instead of the space-wasting smallest-order fallback.
> Maybe we could map some GFP flag to indicate that.

For example, if you create a cache with 17KB objects, the slab subsystem 
will pad it up to 32KB. You are wasting almost 1/2 memory, but the 
allocation is realiable and it won't fail.

If you use order higher than 32KB, you get less wasted memory, but you 
also get random -ENOMEMs (yes, we had a problem in dm-thin that it was 
randomly failing during initialization due to 64KB allocation).

Mikulas

  parent reply	other threads:[~2018-04-17 17:27 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 [this message]
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
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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