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Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
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To:     Peter Zijlstra <peterz@infradead.org>,
        Alex Kogan <alex.kogan@oracle.com>
Cc:     linux@armlinux.org.uk, mingo@redhat.com, will.deacon@arm.com,
        arnd@arndb.de, longman@redhat.com, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        guohanjun@huawei.com, jglauber@marvell.com,
        steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
        dave.dice@oracle.com
References: <20191230194042.67789-1-alex.kogan@oracle.com>
 <20191230194042.67789-5-alex.kogan@oracle.com>
 <20200121132949.GL14914@hirez.programming.kicks-ass.net>
 <20200121135034.GA14946@hirez.programming.kicks-ass.net>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <e10414a6-dbfc-a666-18b8-a0499c93a203@redhat.com>
Date:   Tue, 21 Jan 2020 22:19:32 +0100
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On 1/21/20 2:50 PM, Peter Zijlstra wrote:
> On Tue, Jan 21, 2020 at 02:29:49PM +0100, Peter Zijlstra wrote:
>> On Mon, Dec 30, 2019 at 02:40:41PM -0500, Alex Kogan wrote:
>>
>>> +/*
>>> + * Controls the threshold for the number of intra-node lock hand-offs before
>>> + * the NUMA-aware variant of spinlock is forced to be passed to a thread on
>>> + * another NUMA node. By default, the chosen value provides reasonable
>>> + * long-term fairness without sacrificing performance compared to a lock
>>> + * that does not have any fairness guarantees. The default setting can
>>> + * be changed with the "numa_spinlock_threshold" boot option.
>>> + */
>>> +int intra_node_handoff_threshold __ro_after_init = 1 << 16;
>> There is a distinct lack of quantitative data to back up that
>> 'reasonable' claim there.
>>
>> Where is the table of inter-node latencies observed for the various
>> values tested, and on what criteria is this number deemed reasonable?
>>
>> To me, 64k lock hold times seems like a giant number, entirely outside
>> of reasonable.
> Daniel, IIRC you just did a paper on constructing worst case latencies
> from measuring pieces. Do you have data on average lock hold times?
> 

I am still writing the paper, but I do not have the (avg) lock times. It is it
is in the TODO list, though!

-- Daniel