linux-kernel - Re: [v1 0/5] parallelized "struct page" zeroing

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Message-Id: <20170323.163520.123614131649571916.davem@davemloft.net>
Date:   Thu, 23 Mar 2017 16:35:20 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     willy@...radead.org
Cc:     pasha.tatashin@...cle.com, linux-kernel@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.or
Subject: Re: [v1 0/5] parallelized "struct page" zeroing

From: Matthew Wilcox <willy@...radead.org>
Date: Thu, 23 Mar 2017 16:26:38 -0700

> On Thu, Mar 23, 2017 at 07:01:48PM -0400, Pavel Tatashin wrote:
>> When deferred struct page initialization feature is enabled, we get a
>> performance gain of initializing vmemmap in parallel after other CPUs are
>> started. However, we still zero the memory for vmemmap using one boot CPU.
>> This patch-set fixes the memset-zeroing limitation by deferring it as well.
>> 
>> Here is example performance gain on SPARC with 32T:
>> base
>> https://hastebin.com/ozanelatat.go
>> 
>> fix
>> https://hastebin.com/utonawukof.go
>> 
>> As you can see without the fix it takes: 97.89s to boot
>> With the fix it takes: 46.91 to boot.
> 
> How long does it take if we just don't zero this memory at all?  We seem
> to be initialising most of struct page in __init_single_page(), so it
> seems like a lot of additional complexity to conditionally zero the rest
> of struct page.

Alternatively, just zero out the entire vmemmap area when it is setup
in the kernel page tables.