[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] blkback global resources
>>> On 27.03.12 at 14:34, Wei Liu <wei.liu2@xxxxxxxxxx> wrote: > On Tue, 2012-03-27 at 11:22 +0100, Jan Beulich wrote: >> >>> On 27.03.12 at 11:41, Wei Liu <wei.liu2@xxxxxxxxxx> wrote: >> > On Mon, 2012-03-26 at 17:20 +0100, Ian Campbell wrote: >> >> 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. >> >> > >> > >> > What's your main concern on QoS? Lock contention? Starvation? Or any >> > other things? >> >> However you want to put it. Prior to the multi-page ring patches, we >> have 64 pending requests (global) and 32 ring entries. Obviously, >> bumping the ring size just to order 1 will already bring the number of >> possible in-flight entries per device on par with those in-flight across >> all devices. So _if_ someone really determined that a multi-page ring >> helps performance, I wonder whether that was with manually >> adjusted global pending request values (not said anywhere) or with >> just a single frontend (not very close to real world scenarios). >> > > Just to be precise, bumping order to 1 makes ring entries more than 64. Iirc order 0 -> 32 entries, order 1 -> 64 entries. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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