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From: Michal Hocko <mhocko@suse.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-mm@kvack.org, Daniel Jordan <daniel.m.jordan@oracle.com>,
	Zi Yan <ziy@nvidia.com>, John Hubbard <jhubbard@nvidia.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Oscar Salvador <osalvador@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC V2] mm/vmstat: Add events for HugeTLB migration
Date: Mon, 5 Oct 2020 08:05:42 +0200	[thread overview]
Message-ID: <20201005060542.GM4555@dhcp22.suse.cz> (raw)
In-Reply-To: <fa6a7ea8-df58-c692-0317-686227de54ca@arm.com>

On Mon 05-10-20 07:59:12, Anshuman Khandual wrote:
> 
> 
> On 10/02/2020 05:34 PM, Michal Hocko wrote:
> > On Wed 30-09-20 11:30:49, Anshuman Khandual wrote:
> >> Add following new vmstat events which will track HugeTLB page migration.
> >>
> >> 1. HUGETLB_MIGRATION_SUCCESS
> >> 2. HUGETLB_MIGRATION_FAILURE
> >>
> >> It follows the existing semantics to accommodate HugeTLB subpages in total
> >> page migration statistics. While here, this updates current trace event
> >> 'mm_migrate_pages' to accommodate now available HugeTLB based statistics.
> > What is the actual usecase? And how do you deal with the complexity
> > introduced by many different hugetlb page sizes. Really, what is the
> > point to having such a detailed view on hugetlb migration?
> >
> 
> It helps differentiate various page migration events i.e normal, THP and
> HugeTLB and gives us more reliable and accurate measurement. Current stats
> as per PGMIGRATE_SUCCESS and PGMIGRATE_FAIL are misleading, as they contain
> both normal and HugeTLB pages as single entities, which is not accurate.

Yes this is true. But why does it really matter? Do you have a specific
example?

> After this change, PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will contain page
> migration statistics in terms of normal pages irrespective of whether any
> previous migrations until that point involved normal pages, THP or HugeTLB
> (any size) pages. At the least, this fixes existing misleading stats with
> PGMIGRATE_SUCCESS and PGMIGRATE_FAIL.
> 
> Besides, it helps us understand HugeTLB migrations in more detail. Even
> though HugeTLB can be of various sizes on a given platform, these new
> stats HUGETLB_MIGRATION_SUCCESS and HUGETLB_MIGRATION_FAILURE give enough
> overall insight into HugeTLB migration events.

While true this all is way too vague to add yet another imprecise
counter.

-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2020-10-05  6:05 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-30  6:00 [RFC V2] mm/vmstat: Add events for HugeTLB migration Anshuman Khandual
2020-09-30  7:46 ` Oscar Salvador
2020-09-30 10:02   ` Anshuman Khandual
2020-09-30 10:33     ` Oscar Salvador
2020-10-01 22:16 ` Mike Kravetz
2020-10-02 12:04 ` Michal Hocko
2020-10-05  2:29   ` Anshuman Khandual
2020-10-05  6:05     ` Michal Hocko [this message]
2020-10-06  2:56       ` Anshuman Khandual
2020-10-06  7:54         ` Michal Hocko

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