linux-kernel - Re: scalability regressions related to hugetlb

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Date:   Fri, 25 Mar 2022 13:33:03 +0000
From:   Ray Fucillo <Ray.Fucillo@...ersystems.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: scalability regressions related to hugetlb_fault() changes

> On Mar 25, 2022, at 12:40 AM, Mike Kravetz <mike.kravetz@...cle.com> wrote:
> 
> I will continue to look at this.  A quick check of the fork code shows the
> semaphore held in read mode for the duration of the page table copy.

Thank you for looking into it.  

As a side note about fork() for context, and not to distract from the 
regression at hand...  There's some history here where we ran into problems 
circa 2005 where fork time went linear with the size of shared memory, and 
that was resolved by letting the pages fault in the child.  This was when
hugetlb was pretty new (and not used by us) and I see now that the fix
explicitly excluded hugetlb.  Anyway, we now mostly use vfork(), only fork()
in some special cases, and improving just fork wouldn't fix the scalability
regression for us.  But, it does sound like fork() time might be getting 
large again now that everyone is using very large shared segments with 
hugetlb, but generally haven't switched to 1GB pages.  That old thread is: 
https://lkml.org/lkml/2005/8/24/190