Re: [Qemu-devel] [PATCH v4 5/5] qemu-iotests: Add 093 for IO throttling

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jun 05, 2014 at 04:47:46PM +0800, Fam Zheng wrote:
> +    def setUp(self):
> +        qemu_img('create', '-f', iotests.imgfmt, test_img, "1G")
> +        #self.vm = iotests.VM().add_drive(test_img, "bps=1024,bps_max=1")

Commented out lines should be dropped

> +        self.vm = iotests.VM().add_drive(test_img)
> +        self.vm.launch()
> +
> +    def tearDown(self):
> +        self.vm.shutdown()
> +        os.remove(test_img)
> +
> +    def do_test_throttle(self, seconds=10, **limits):
> +        def check_limit(limit, num):
> +            # IO throttling algorithm is discrete, allow 10% error so the test
> +            # is more deterministic
> +            return limit == 0 or num < seconds * limit * 1.1
> +
> +        nsec_per_sec = 1000000000
> +
> +        limits['bps_max'] = 1
> +        limits['iops_max'] = 1
> +
> +        # Enqueue many requests to throttling queue

This comment is wrong, it actually happens further down

> +        result = self.vm.qmp("block_set_io_throttle", conv_keys=False, **limits)
> +        self.assert_qmp(result, 'return', {})
> +
> +        # Set vm clock to a known value
> +        ns = nsec_per_sec
> +        self.vm.qtest_cmd("clock_step %d" % ns)
> +
> +        # Append many requests into the throttle queue
> +        # They drain bps_max and iops_max
> +        # The rest requests won't get executed until qtest clock is driven
> +        for i in range(1000):
> +            self.vm.hmp_qemu_io("drive0", "aio_read -a -q 0 512")
> +            self.vm.hmp_qemu_io("drive0", "aio_write -a -q 0 512")
> +
> +        start_rd_bytes, start_rd_iops, start_wr_bytes, start_wr_iops = self.blockstats('drive0')
> +
> +        ns += seconds * nsec_per_sec
> +        self.vm.qtest_cmd("clock_step %d" % ns)
> +        # wait for a while to let requests take off
> +        time.sleep(1)

This is not a reliable testing approach.  If the system is under heavy
load maybe only a few requests completed.  We don't know whether that is
due to I/O throttling or not.

A reliable test would not perform real disk I/O so the test is
independent of disk/system speed.  And it would not use time.sleep(1) to
"wait" since there is no guarantee that anything happened in the
meantime.

Do you think this can be improved?

> +        end_rd_bytes, end_rd_iops, end_wr_bytes, end_wr_iops = self.blockstats('drive0')
> +
> +        rd_bytes = end_rd_bytes - start_rd_bytes
> +        rd_iops = end_rd_iops - start_rd_iops
> +        wr_bytes = end_wr_bytes - start_wr_bytes
> +        wr_iops = end_wr_iops - start_wr_iops
> +
> +        assert check_limit(limits['bps'], rd_bytes)
> +        assert check_limit(limits['bps_rd'], rd_bytes)
> +        assert check_limit(limits['bps'], wr_bytes)
> +        assert check_limit(limits['bps_wr'], wr_bytes)
> +        assert check_limit(limits['iops'], rd_iops)
> +        assert check_limit(limits['iops_rd'], rd_iops)
> +        assert check_limit(limits['iops'], wr_iops)
> +        assert check_limit(limits['iops_wr'], wr_iops)

Please use TestCase.assert*() methods instead of plain assert.  They
produce humand-readable error messages including the failing values.

> +
> +    def test_bps(self):
> +        self.do_test_throttle(**{
> +            'device': 'drive0',
> +            'bps': 1000,
> +            'bps_rd': 0,
> +            'bps_wr': 0,
> +            'iops': 0,
> +            'iops_rd': 0,
> +            'iops_wr': 0,
> +            })

Keyword argument syntax is more concise:

self.do_test_throttle(device='drive0',
                      bps=1000,
		      bps_rd=0,
		      ...)

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