Re: [Qemu-devel] [PATCH v5 04/12] rdma: introduce qemu_get_max_size()

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 04/22/2013 04:20 PM, Eric Blake wrote:
> On 04/21/2013 03:17 PM, mrhines@linux.vnet.ibm.com wrote:
>> From: "Michael R. Hines" <mrhines@us.ibm.com>
>>
>> This functions allows you to perform your own per-QEMUFileOps
>> calculation for the value of 'max_size'.
>>
>> For RDMA, this calculation artificially limits migration throughput
>> and needs to be done differently for high-throughput links.
>>
>> Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
>> ---
>>   
>> +size_t qemu_get_max_size(QEMUFile *f, uint64_t transferred_bytes,
>> +                         uint64_t time_spent, uint64_t max_downtime)
>> +{
>> +    if (time_spent) {
>> +        mbps = (((double) transferred_bytes * 8.0) /
>> +            ((double) time_spent / 1000.0)) / 1000.0 / 1000.0;
>> +    } else {
>> +        mbps = -1.0;
>> +    }
>> +
>> +    if (f->ops->get_max_size) {
>> +        return f->ops->get_max_size(f, f->opaque,
>> +            transferred_bytes, time_spent, max_downtime);
>> +    }
>> +
>> +    return ((double) (transferred_bytes / time_spent)) *
>> +                    max_downtime / 1000000;
> Cast to double is too late; you've already suffered from integer
> division truncation when you compute (transferred_bytes/time_spent).
>

Good catch, thank you.