Re: [Xen-devel] [PATCH 00/25 v7] SBSA UART emulation support in Xen

[Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>]

Hi Wei,

On 9 August 2017 at 16:33, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> On Wed, Aug 09, 2017 at 04:28:14PM +0530, Bhupinder Thakur wrote:
>> Hi Julien,
>>
>> Thanks for the testing.
>>
>> On 8 August 2017 at 21:29, Julien Grall <julien.grall@xxxxxxx> wrote:
>> > Hi Bhupinder,
>> >
>> > I gave another and I have a couple of comments.
>> >
>> > Booting Linux with earlycon enabled take quite a while. I can see the
>> > characters coming slower than on the minitel. It seems to be a bit better
>> > after switching off the bootconsole. Overall Linux is taking ~20 times to
>> > boot with pl011 vs HVC console.
>> >
>> > I do agree that pl011 is emulated and therefore you have to trap after each
>> > character. But 20 times sounds far too much.
>> >
>> I think this slowness could be due to ratelimiting of the pl011 events
>> in xenconosle. Currently, the rate limit is
>> set to 30 events per 200 msecs (see RATE_LIMIT_ALLOWANCE/RATE_LIMIT_PERIOD).
>>
>> I increased the rate limit to 600 events (30 * 20) per 200 msecs. With
>> this change,
>> I see that the the find command is running faster and smoother.
>> Earlier the find output would be jerky.
>
> Please consider batching the events instead of bumping the limit.

If I try to batch the events then it may delay per character
processing (if no further characters are coming) since we will wait
for more events to come to batch them together. By keeping the rate
limit high, it is ensured that it can handle large burst of events. If
there is sporadic data then the rate limit is not hit.

I am thinking of making the rate limit configuration per console.

Regards,
Bhupinder

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