From: Michael Ellerman <mpe@ellerman•id.au>
To: David Hildenbrand <david@redhat•com>,
"Aneesh Kumar K.V" <aneesh.kumar@linux•ibm.com>,
linuxppc-dev@lists•ozlabs.org, npiggin@gmail•com,
christophe.leroy@csgroup•eu
Cc: foraker1@llnl•gov
Subject: Re: [PATCH v2 2/2] powerpc/mm: Add memory_block_size as a kernel parameter
Date: Tue, 20 Jun 2023 22:35:44 +1000 [thread overview]
Message-ID: <87mt0upazj.fsf@mail.lhotse> (raw)
In-Reply-To: <853eae60-b92b-9284-e24d-564429aba8c1@redhat.com>
David Hildenbrand <david@redhat•com> writes:
> On 09.06.23 08:08, Aneesh Kumar K.V wrote:
>> Certain devices can possess non-standard memory capacities, not constrained
>> to multiples of 1GB. Provide a kernel parameter so that we can map the
>> device memory completely on memory hotplug.
>
> So, the unfortunate thing is that these devices would have worked out of
> the box before the memory block size was increased from 256 MiB to 1 GiB
> in these setups. Now, one has to fine-tune the memory block size. The
> only other arch that I know, which supports setting the memory block
> size, is x86 for special (large) UV systems -- and at least in the past
> 128 MiB vs. 2 GiB memory blocks made a performance difference during
> boot (maybe no longer today, who knows).
>
>
> Obviously, less tunable and getting stuff simply working out of the box
> is preferable.
>
> Two questions:
>
> 1) Isn't there a way to improve auto-detection to fallback to 256 MiB in
> these setups, to avoid specifying these parameters?
>
> 2) Is the 256 MiB -> 1 GiB memory block size switch really worth it? On
> x86-64, experiments (with direct map fragmentation) showed that the
> effective performance boost is pretty insignificant, so I wonder how big
> the 1 GiB direct map performance improvement is.
The other issue is simply the number of sysfs entries.
With 64TB of memory and a 256MB block size you end up with ~250,000
directories in /sys/devices/system/memory.
cheers
next prev parent reply other threads:[~2023-06-20 12:36 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-09 6:08 [PATCH v2 1/2] powerpc/mm: Cleanup memory block size probing Aneesh Kumar K.V
2023-06-09 6:08 ` [PATCH v2 2/2] powerpc/mm: Add memory_block_size as a kernel parameter Aneesh Kumar K.V
2023-06-13 20:06 ` Reza Arbab
2023-06-19 10:35 ` David Hildenbrand
2023-06-19 16:17 ` Aneesh Kumar K.V
2023-06-19 16:28 ` David Hildenbrand
2023-06-20 12:35 ` Michael Ellerman [this message]
2023-06-20 12:53 ` David Hildenbrand
2023-06-13 19:53 ` [PATCH v2 1/2] powerpc/mm: Cleanup memory block size probing Reza Arbab
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