linux-kernel - Re: Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage

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Date:	Thu, 18 Apr 2013 16:05:19 +0800
From:	zhang.yi20@....com.cn
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>, Dave Hansen <dave@...1.net>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Re: [PATCH] futex: bugfix for futex-key conflict when futex use
 hugepage

Darren Hart <dvhart@...ux.intel.com> wrote on 2013/04/17 23:51:36:

> On 04/17/2013 08:26 AM, Dave Hansen wrote:
> > On 04/17/2013 07:18 AM, Darren Hart wrote:
> >>>> This also needs a comment in futex.h describing the usage of the
> >>>> offset field in union futex_key as well as above get_futex_key
> >>>> describing the key for shared mappings.
> >>>>
> >>> As far as I know , the max size of one hugepage is 1 GBytes for 
> >>> x86 cpu. Can some other cpus support greater hugepage even more 
> >>> than 4 GBytes? If so, we can change the type of 'offset' from int 
> >>>  to long to avoid truncating.
> >>
> >> I discussed this with Dave Hansen, on CC, and he thought we needed
> >> 9 bits, so even on x86 32b we should be covered.
> > 
> > I think the problem is actually on 64-bit since you still only have
> > 32-bits in an 'int' there.
> > 
> > I guess it's remotely possible that we could have some
> > mega-super-huge-gigantic pages show up in hardware some day, or that
> > somebody would come up with software-only one.  I bet there's a lot
> > more code that will break in the kernel than this futex code, though.
> > 
> > The other option would be to start #defining some build-time constant
> > for what the largest possible huge page size is, then BUILD_BUG_ON()
> > it.
> >
> > Or you can just make it a long ;)
> 
> If we make it a long I'd want to see futextest performance tests before
> and after. Messing with the futex_key has been known to have bad results
> in the past :-)
> 
> -- 
 
I have run futextest/performance/futex_wait for testing, 5 times before 
make it long:
futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10215 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 9862 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10081 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10060 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10081 Kiter/s


And 5 times after make it long:
futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 9940 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10204 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 9901 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10152 Kiter/s

futex_wait: Measure FUTEX_WAIT operations per second
        Arguments: iterations=100000000 threads=256
Result: 10060 Kiter/s


Seems OK, is it?
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