From: Yang Yingliang <yangyingliang@huawei•com>
To: David Laight <David.Laight@ACULAB•COM>, <davem@davemloft•net>,
<netdev@vger•kernel.org>
Cc: <eric.dumazet@gmail•com>, <brouer@redhat•com>,
<jpirko@redhat•com>, <jbrouer@redhat•com>
Subject: Re: [PATCH net v6 1/2] net: sched: tbf: fix the calculation of max_size
Date: Mon, 9 Dec 2013 10:26:37 +0800 [thread overview]
Message-ID: <52A52A5D.5060601@huawei.com> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B7468@saturn3.aculab.com>
On 2013/12/6 18:56, David Laight wrote:
>> From: Yang Yingliang
> ...
>>
>> +/* Time to Length, convert time in ns to length in bytes
>> + * to determinate how many bytes can be sent in given time.
>> + */
>> +static u64 psched_ns_t2l(const struct psched_ratecfg *r,
>> + u64 time_in_ns)
>> +{
>> + /* The formula is :
>> + * len = (time_in_ns * r->rate_bytes_ps) / NSEC_PER_SEC
>> + */
>> + u64 len = time_in_ns * r->rate_bytes_ps;
>> +
>> + do_div(len, NSEC_PER_SEC);
>
> You are multiplying two values then dividing by 10**9
> I'd guess that the intermediate value might exceed 2**64.
I thought the max value of len is burst which is a type of u32 sent
userland, so the max value of (time_in_ns * r->rate_bytes_ps)
should be 64K*(10**9).
Hmm, maybe I should do something in this helper to avoid overflow.
>
>> + if (unlikely(r->linklayer == TC_LINKLAYER_ATM))
>> + len = (len / 53) * 48;
>
> You probably want to do the multiply first.
> But why not scale rate_bytes_ps instead.
>
>> + if (len > r->overhead)
>> + len -= r->overhead;
>> + else
>> + len = 0;
>> +
>> + return len;
>> +}
>
> Personally I'd work out how much time you have to send each byte.
> So if you want a rate of 1MB/sec you have a 'time cost' per byte of 1000ns.
> The cost of sending a packet is simply the length multiplied by this cost.
> To work out whether a packet can be sent you have a credit variable that
> tracks current time.
> If 'credit > now' the packet can't be sent, queue and schedule a wakeup.
> if 'credit + backlog < now' set credit = now - backlog.
> if 'credit <= now' send the packet and add the packet's 'cost' to 'credit'.
>
> In the non-ratelimited case this is almost no work.
>
> You'd probably need to work in 1/1024ns time units and/or blocks of 16 bytes.
>
> David
>
>
>
>
> .
>
next prev parent reply other threads:[~2013-12-09 2:29 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-06 7:00 [PATCH net v6 0/2] net: sched: fix two issues Yang Yingliang
2013-12-06 7:00 ` [PATCH net v6 1/2] net: sched: tbf: fix the calculation of max_size Yang Yingliang
2013-12-06 10:56 ` David Laight
2013-12-09 2:26 ` Yang Yingliang [this message]
2013-12-09 3:26 ` Yang Yingliang
2013-12-09 10:07 ` David Laight
2013-12-09 12:21 ` Yang Yingliang
2013-12-09 13:10 ` [PATCH RFC ] " Yang Yingliang
2013-12-09 15:12 ` Eric Dumazet
2013-12-10 2:29 ` Yang Yingliang
2013-12-10 2:39 ` Eric Dumazet
2013-12-09 13:11 ` [PATCH net v6 1/2] " David Laight
2013-12-10 2:04 ` Yang Yingliang
2013-12-06 7:00 ` [PATCH net v6 2/2] net: sched: htb: fix the calculation of quantum Yang Yingliang
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