FT> Yeah, I think that there is no performance difference under real FT> workloads (not synthetic benchmark workloads). So dbench is too synthetic? Performance with guest domains has been quite good in my experience. FT> Right, I should have said that the major advantage of ring buffer FT> is communication without system calls. That's definitely possible, but if the remap cache reduces the amount of communication with userspace, then you also save the context switch to map the data each time. I know that blktap does not seem to suffer much of a performance hit here, so it may be lost in the noise of a domain switch. What about native performance? I intend for dm-userspace to be useful outside the realm of Xen :) FT> I've not tried the origianl dm-userspace. Then why do you claim that performance suffers because of the use of syscalls? FT> You have all the equipment, so can you do a performance FT> comparison? Yes, I can. FT> I guess that the results of Xen blktap and blkback drivers have FT> told us the expected performance differences. Is it not possible that blktap performs well for Xen because I/O latency is hidden by domain switches? I would think that on a single processor vanilla linux system that switching to userspace for every single map would not be ideal. -- Dan Smith IBM Linux Technology Center Open Hypervisor Team email: danms@xxxxxxxxxx
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