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[Xen-devel] blkback global resources

  • To: "xen-devel" <xen-devel@xxxxxxxxxxxxx>
  • From: "Jan Beulich" <JBeulich@xxxxxxxx>
  • Date: Mon, 26 Mar 2012 16:56:36 +0100
  • Delivery-date: Mon, 26 Mar 2012 15:56:10 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

All the resources allocated based on xen_blkif_reqs are global in
blkback. While (without having measured anything) I think that this
is bad from a QoS perspective (not the least implied from a warning
issued by Citrix'es multi-page-ring patches:

        if (blkif_reqs < BLK_RING_SIZE(order))
                printk(KERN_WARNING "WARNING: "
                       "I/O request space (%d reqs) < ring order %ld, "
                       "consider increasing %s.reqs to >= %ld.",
                       blkif_reqs, order, KBUILD_MODNAME,

indicating that this _is_ a bottleneck), I'm otoh hesitant to convert
this to per-instance allocations, as the amount of memory taken
away from Dom0 for this may be not insignificant when there are
many devices.

Does anyone have an opinion here, in particular regarding the
original authors' decision to make this global vs. the apparently
made observation (by Daniel Stodden, the author of said patch,
who I don't have any current email of to ask directly), but also
in the context of multi-page rings, the purpose of which is to
allow for larger amounts of in-flight I/O?

Thanks, Jan

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