Re: [Xen-devel] blkback global resources

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-03-26 at 16:56 +0100, Jan Beulich wrote:
> 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,
>                      roundup_pow_of_two(BLK_RING_SIZE(order)));
> 
> 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?

Not really much to say other than we (well, mostly Wei Liu) have a
similar issue with netback too. That used to have a global pool, then a
pool-per-worker thread. When Wei added thread-per-vif support he solved
this by adding a "page pool" which handles allocations. Possibly this
could grow some sort of fairness etc nobs and be shared with blkback?

Ian.


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