From: Thomas Gleixner <tglx@linutronix.de>
To: 20240621164406.256314-1-kirill.shutemov@linux.intel.com,
kirill.shutemov@linux.intel.com
Cc: ardb@kernel.org, bp@alien8.de, brijesh.singh@amd.com,
corbet@lwn.net, dave.hansen@linux.intel.com, hpa@zytor.com,
jan.kiszka@siemens.com, jgross@suse.com, kbingham@kernel.org,
linux-doc@vger.kernel.org, linux-efi@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
luto@kernel.org, michael.roth@amd.com, mingo@redhat.com,
peterz@infradead.org, rick.p.edgecombe@intel.com,
sandipan.das@amd.com, thomas.lendacky@amd.com, x86@kernel.org
Subject: Re: [PATCH 0/3] x86: Make 5-level paging support unconditional for x86-64
Date: Wed, 31 Jul 2024 11:15:05 +0200 [thread overview]
Message-ID: <87wml16hye.ffs@tglx> (raw)
In-Reply-To: <80734605-1926-4ac7-9c63-006fe3ea6b6a@amd.com>
On Wed, Jul 31 2024 at 14:27, Shivank Garg wrote:
> lmbench:lat_pagefault: Metric- page-fault time (us) - Lower is better
> 4-Level PT 5-Level PT % Change
> THP-never Mean:0.4068 Mean:0.4294 5.56
> 95% CI:0.4057-0.4078 95% CI:0.4287-0.4302
>
> THP-Always Mean: 0.4061 Mean: 0.4288 % Change
> 95% CI: 0.4051-0.4071 95% CI: 0.4281-0.4295 5.59
>
> Inference:
> 5-level page table shows increase in page-fault latency but it does
> not significantly impact other benchmarks.
5% regression on lmbench is a NONO.
5-level page tables add a cost in every hardware page table walk. That's
a matter of fact and there is absolutely no reason to inflict this cost
on everyone.
The solution to this to make the 5-level mechanics smarter by evaluating
whether the machine has enough memory to require 5-level tables and
select the depth at boot time.
Thanks,
tglx
next prev parent reply other threads:[~2024-07-31 9:15 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-07-31 8:57 Shivank Garg
2024-07-31 9:15 ` Thomas Gleixner [this message]
2024-07-31 11:11 ` Peter Zijlstra
2024-07-31 11:36 ` Kirill A. Shutemov
2024-07-31 11:40 ` Peter Zijlstra
2024-07-31 17:45 ` Shivank Garg
2024-10-31 15:36 ` Dave Hansen
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