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Re: [Xen-devel] Question about high CPU load during iperf ethernet testing

On Mon, 22 Sep 2014, Iurii Konovalenko wrote:
> Hello, all!
> I am running iperf ethernet tests on DRA7XX_EVM board (OMAP5).
> Xen version is 4.4.
> I run only Linux (kernel 3.8) as Dom0, no other active domains (For clear 
> tests results I decided not to start DomU).
> iperf server is started on host, iperf client is started on board with 
> command line "iperf -c -w 256k -m
> -f M -d -t 60".

Just to double check: you are running the iperf test in Dom0, correct?

> During test I studied CPU load with top tool on Dom0, and saw, that one VCPU 
> is totally loaded, spending about 50% in
> software IRQs, and 50% in system.
> Running the same test on clear Linux without Xen, I saw that CPU load is 
> about 2-4%.
> I decided to debug a bit, so I used "({register uint64_t _r; asm 
> volatile("mrrc " "p15, 0, %0, %H0, c14" ";" : "=r"
> (_r)); _r; })" command to read timer counter before and after operations I 
> want to test.
> In such way I've found, that most time of CPU is spent in functions 
> enable_irq/disable_irq_nosync and
> spin_lock_irqsave/spin_unlock_irqrestore (mostly in "mrsÂÂÂ %0, cpsrÂÂÂ @ 
> arch_local_irq_save"/"msrÂÂÂ cpsr_c, %0ÂÂÂ @
> local_irq_restore"). When running without Xen it should not take so much time.

There is nothing Xen specific in the Linux ARM implementation of
spin_lock_irqsave/spin_unlock_irqrestore and

> So, could anyone explain me some questions:
> 1. Is it normal behaviour?

No, it is not normal.
Assuming that you assign all the memory to Dom0 and as many vcpu as
physical cpus on your platform then you should get the same numbers as

> 2. Does hypervisor trap cpsr register? I suppose, that hypervisor trap access 
> to cpsr register, that leads to
> additional overhead, but I can't find place in sources where it happens.

We don't trap cpsr.

> Thank you in advance.
> Best regards.
> Iurii KonovalenkoÂ| Senior Software Engineer
> GlobalLogic
> PÂ+3.8044.492.9695 M +38.099.932.2909ÂÂ
> SÂyufuntik
> www.globallogic.com
> http://www.globallogic.com/email_disclaimer.txt
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