[Xen-devel] blkfront resource use

["Jan Beulich" <JBeulich@xxxxxxxx>]

Konrad, Roger,

we've recently had a report of stuck I/O (with slightly over a hundred
frontend instances in a single guest), which turned out to be a simple
lack of configured grants on the system. This raises two questions:

1) Shouldn't blkfront cope with this situation, by throttling I/O (but not
getting stuck), perhaps by forcing requests to be split (or splitting
them itself) if gnttab_alloc_grant_references() continues to fail over
a measurable time period?

2) Isn't the current grant hunger of blkfront a little extreme? An
instance with all defaults can consume 1k grants (32 segs times
32 reqs), but the scaling with ring size, queue count, and number
of segs can easily get this to 64k or more. A default configured
host, however, allows only for 128k grants (256 frames each
holding 512 entries). Interestingly I've found 
https://groups.google.com/forum/#!topic/linux.kernel/N6Q171xkIkM
when looking around - is there a reason this or something similar
never made it into the driver? Without such adjustment a single
spike in I/O can lead to a significant amount of grants to be "lost"
in the queue of a single frontend instance.

Thanks, Jan


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