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Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default
Date: Tue, 28 Nov 2017 23:20:27 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/28/2017 10:58 PM, Eduardo Habkost wrote:
> Hi,
> On Fri, Nov 24, 2017 at 04:26:50PM +0300, Denis Plotnikov wrote:
>> Commit 14c985cffa "target-i386: present virtual L3 cache info for vcpus"
>> introduced and set by default exposing l3 to the guest.
>> The motivation behind it was that in the Linux scheduler, when waking up
>> a task on a sibling CPU, the task was put onto the target CPU's runqueue
>> directly, without sending a reschedule IPI.  Reduction in the IPI count
>> led to performance gain.
>> However, this isn't the whole story.  Once the task is on the target
>> CPU's runqueue, it may have to preempt the current task on that CPU, be
>> it the idle task putting the CPU to sleep or just another running task.
>> For that a reschedule IPI will have to be issued, too.  Only when that
>> other CPU is running a normal task for too little time, the fairness
>> constraints will prevent the preemption and thus the IPI.
>> This boils down to the improvement being only achievable in workloads
>> with many actively switching tasks.  We had no access to the
>> (proprietary?) SAP HANA benchmark the commit referred to, but the
>> pattern is also reproduced with "perf bench sched messaging -g 1"
>> on 1 socket, 8 cores vCPU topology, we see indeed:
>> l3-cache     #res IPI /s     #time / 10000 loops
>> off          560K            1.8 sec
>> on           40K             0.9 sec
>> Now there's a downside: with L3 cache the Linux scheduler is more eager
>> to wake up tasks on sibling CPUs, resulting in unnecessary cross-vCPU
>> interactions and therefore exessive halts and IPIs.  E.g. "perf bench
>> sched pipe -i 100000" gives
>> l3-cache     #res IPI /s     #HLT /s         #time /100000 loops
>> off          200 (no K)      230             0.2 sec
>> on           400K            330K            0.5 sec
>> In a more realistic test, we observe 15% degradation in VM density
>> (measured as the number of VMs, each running Drupal CMS serving 2 http
>> requests per second to its main page, with 95%-percentile response
>> latency under 100 ms) with l3-cache=on.
>> We think that mostly-idle scenario is more common in cloud and personal
>> usage, and should be optimized for by default; users of highly loaded
>> VMs should be able to tune them up themselves.
> There's one thing I don't understand in your test case: if you
> just found out that Linux will behave worse if it assumes that
> the VCPUs are sharing a L3 cache, why are you configuring a
> 8-core VCPU topology explicitly?
> Do you still see a difference in the numbers if you use "-smp 8"
> with no "cores" and "threads" options?
This is quite simple. A lot of software licenses are bound to the amount
of CPU __sockets__. Thus it is mandatory in a lot of cases to set topology
with 1 socket/xx cores to reduce the amount of money necessary to
be paid for the software.


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