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Re: [PATCH] Optimize libc_lock_lock for MIPS XLP.
- From: Tom de Vries <Tom_deVries at mentor dot com>
- To: Chris Metcalf <cmetcalf at tilera dot com>
- Cc: Maxim Kuvyrkov <maxim at codesourcery dot com>, "Joseph S. Myers" <joseph at codesourcery dot com>, GLIBC Devel <libc-alpha at sourceware dot org>, <libc-ports at sourceware dot org>, Tom de Vries <vries at codesourcery dot com>
- Date: Fri, 6 Jul 2012 21:41:41 +0200
- Subject: Re: [PATCH] Optimize libc_lock_lock for MIPS XLP.
- References: <FC4EF172-B43E-4298-A2E9-681FA28650DB@mentor.com> <4FD9DB74.8080905@tilera.com> <40CBC472-71CC-4FF3-A452-073B76701215@codesourcery.com> <4FDAA190.3050706@tilera.com> <C8A2E1C0-8B9E-4C07-96F6-3F83C2E88C61@codesourcery.com> <15EB7E17-5692-4221-A1B1-FC16EA236BFF@codesourcery.com> <4FEC94AF.40301@tilera.com>
On 28/06/12 19:30, Chris Metcalf wrote:
> On 6/27/2012 5:45 PM, Maxim Kuvyrkov wrote:
>> On 15/06/2012, at 2:49 PM, Maxim Kuvyrkov wrote:
>>
>>>> On 15/06/2012, at 2:44 PM, Chris Metcalf wrote:
>>>>
>>>>>> On 6/14/2012 9:20 PM, Maxim Kuvyrkov wrote:
>>>> ...
>>>>>>>> As I read it, in case of a contended lock __lll_lock_wait will reset the value of the lock to "2" before calling lll_futex_wait(). I agree that there is a timing window in which the other threads will see a value of the lock greater than "2", but the value will not get as high as hundreds or billions as it will be constantly reset to "2" in atomic_exchange in lll_lock_wait().
>>>>>>>>
>>>>>>>> I do not see how threads will get into a busywait state, though. Would you please elaborate on that?
>>>>>>
>>>>>> You are correct. I was thinking the that the while loop had a cmpxchg, but
>>>>>> since it's just a straight-up exchange, the flow will be something like:
>>>>>>
>>>>>> - Fail to initially call lll_futex_wait() if the lock is contended
>>>>>> - Fall through to while loop
>>>>>> - Spin as long as the lock is contended enough that *futex > 2
>>>>>> - Enter futex_wait
>>>>>>
>>>>>> So a little busy under high contention, but probably settles out reasonably
>>>>>> well.
>>>>
>> Attached is an improved patch that also optimizes __libc_lock_trylock using XLP's atomic instructions.
>>
>> The patch also removes unnecessary indirection step represented by new macros lll_add_lock, which is then used to define __libc_lock_lock, and defines __libc_lock_lock and __libc_lock_trylock directly in lowlevellock.h . This makes changes outside of ports/ trivial.
>>
>> Tested on MIPS XLP with no regressions. OK to apply for 2.17?
>
> It looks OK to me. I would want someone else to sign off on it before
> applying to 2.17.
>
Chris,
I cannot sign off on this, but I reviewed the current patch as well and it looks
ok to me too.
Thanks,
- Tom