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[Xen-devel] (Possible) Inaccurate accounting in Xen's bandwidth control

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] (Possible) Inaccurate accounting in Xen's bandwidth control
From: Luwei Cheng <chengluwei@xxxxxxxxx>
Date: Sat, 12 Mar 2011 15:59:01 +0800
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These days I use netperf to evaluate Xen's network bandwidth control.
I found that somehow the bandwidth is always inaccurate (slightly less than the promised bandwidth) 

After reading the source code of Linux-2.6.3** (domain 0), I am a bit curious about the following algorithm.
file: /linux-main-dir/drivers/xen/netback/netback.c
static void tx_add_credit(netif_t *netif)
        unsigned long max_burst, max_credit;

         * Allow a burst big enough to transmit a jumbo packet of up to 128kB.
         * Otherwise the interface can seize up due to insufficient credit.
        max_burst = RING_GET_REQUEST(&netif->tx, netif->tx.req_cons)->size;
        max_burst = min(max_burst, 131072UL);
        max_burst = max(max_burst, netif->credit_bytes);

        /* Take care that adding a new chunk of credit doesn't wrap to zero. */
        max_credit = netif->remaining_credit + netif->credit_bytes;
        if (max_credit < netif->remaining_credit)
                max_credit = ULONG_MAX; /* wrapped: clamp to ULONG_MAX */

        netif->remaining_credit = min(max_credit, max_burst);

Setting: rate=512Kb/s@30ms, Then,
"netif->credit_bytes" will be 1,920
"netif->credit_usecs" will be 30,000
Suppose that at some moment:
"request size" is 1514
"netif->remaining_credit" is 406
Since there's not enough credits for transmitting, the netif
will be delayed for 30,000 usecs to refill credits (using timer).
After the timer wakes up, should the netif get (406+1920) credits, or (1920) credits?  
In my mind, I think it should be fair to let the netif get (406+1920) credits.
However, the above algorithm will eventually give only (1920) credits.

Just wonder whether there's some undocumented consideration to control in this way
(only give 1920 credits)?

Kindly please correct me if my analysis is incorrect.

Thanks for your attention.

Best Regards,
Luwei Cheng
Department of Computer Science
The University of Hong Kong
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