linux-kernel - Re: [patch 1/1] Replace the memtype

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Date:	Fri, 26 Feb 2010 16:52:59 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Robin Holt <holt@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 1/1] Replace the memtype_lock spinlock with an rwlock.

On Fri, 2010-02-26 at 09:42 -0800, Robin Holt wrote:
> While testing an application using the xpmem (out of kernel driver), we
> noticed a significant page fault rate reduction of x86_64 with respect
> to ia64.  For one test running with 256 cpus, one thread per cpu, it
> took one minute, eight seconds for each of the threads to vm_insert_pfn
> 2GB worth of pages.
> 
> The slowdown was tracked to lookup_memtype which acquires the spinlock
> memtype_lock.  This heavily contended lock was slowing down fault time.
> 
> I quickly converted the spinlock to an rwlock.  This greatly improved
> vm_insert_pfn time to 4.3 seconds for the above test.

Acked-by: Suresh Siddha <suresh.b.siddha@...el.com>

> As a theoretical test, I removed the lock around get_page_memtype to see
> what the full impact of the single shared lock actually was.  With that
> change, the vm_insert_pfn time dropped to 1.6 seconds.
> 
> I do not think the current global lock to protect individual pages is
> the "correct" method for protecting get/set of the page_memtype flag.
> It seems like the locking should be located within the function that
> gets and sets the flags and be locked by a per-page lock or protected
> with an atomic update.

I agree. Will you be willing to look into this and post a patch? Or else
I can look at this and post a fix next week.

thanks,
suresh

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