Re: [Qemu-devel] How to measure guest memory access (qemu_ld/qemu_st) time?

[YeongKyoon Lee]

I don't know much of instrumentation approach of Lluis, however, I had roughly estimated guest memory access (qemu_ld/st performance) overhead in legacy way last year.

My idea was indirect performance estimation by measuring the generated host instruction counts of qemu_ld/st and their execution counts.

For measuring execution counts of IR, I had add some legacy instrumentation IR when generating TB IRs in TCG.

 

The result was as follows (qemu_ld/st overhead when Linux of x86 guest booting on QEMU 1.0)

  - 1 qemu_ld/st IR -> 12 host instruction when fast path (TLB hit case)

  - qemu_ld execution count: 5.3%

  - qemu_st execution count: 4.8%

 

Therefore, the performance overhead of qemu_ld/st could be up to about 50% (10% execution count * 12 instruction) using following rough assumptions.

1. Most qemu IRs are translated into one host instruction respectively.

2. The CPU cycles of host instructions are equal.

3. Ignoring TLB miss case.

 

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