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[Xen-devel] doubt regarding IO scheduler and credit scheduling in netback

  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: Kuriakose Mathew <kmathew123@xxxxxxxxx>
  • Date: Fri, 23 Oct 2009 08:46:02 +0530
  • Delivery-date: Thu, 22 Oct 2009 20:16:25 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

With reference to the netback and netfront model of device driver in Xen(specifically Xen 3.1.3 and linux I know it is a bit old but please excuse), I am confused as to where the network IO scheduler fits in. I have read that there are different types of IO schedulers in linux - deadline ,anticipatory, complete fair queueing (cfq )etc. where cfq is the default. Is it only being used in the context of disk IO? or the same thing is used for network IO?

Also (as a related doubt), in netback code(In netback.c net_tx_action ) , I saw some credit scheduling of the network interfaces(netif_t *netif) corresponding to guest domains. So whenever Dom 0 is scheduled ,it does a credit scheduling of the network interfaces corresponding to different DomUs. The credits will get refreshed when the credit_timeout happens. The next network interface(netif) will be scheduled if the credits for the current netif is exhausted. Is my understanding correct ?
What are the other cases when the next netif is scheduled? (one would obviously be the case when there are no packets to send. Any other case?) When I did some experiments, I found that the bandwidth is fairly divided among all the domains. So there should be some mechasim to ensure this.

In this context where does the IO scheduler fits in?
Please explain.

Kuriakose Mathew

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