Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH] cfq-iosched: non-rot devices do not need read queue merging | Date | Tue, 05 Jan 2010 09:58:52 -0500 |
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Corrado Zoccolo <czoccolo@gmail.com> writes:
> On Mon, Jan 4, 2010 at 8:04 PM, Jeff Moyer <jmoyer@redhat.com> wrote: >> Vivek Goyal <vgoyal@redhat.com> writes: >>>> >>> Hi Corrado, >>>> >>> >>>> >>> What's the reason that reads don't benefit from merging queues and hence >>>> >>> merging requests and only writes do on SSD? >>>> >> >>>> >> On SSDs, reads are just limited by the maximum transfer rate, and >>>> >> larger (i.e. merged) reads will just take proportionally longer. >>>> > >>>> > This is simply not true. You can get more bandwidth from an SSD (I just >>>> > checked numbers for 2 vendors' devices) by issuing larger read requests, >>>> > no matter whether the access pattern is sequential or random. >>>> I know, but the performance increase given the size is sublinear, and >>>> the situation here is slightly different. >>>> In order for the requests to be merged, they have to be submitted concurrently. >>>> So you have to compare 2 concurrent requests of size x with one >>>> request of size 2*x (with some CPU overhead). >>>> Moreover, you always pay the CPU overhead, even if you can't do the >>>> merging, and you must be very lucky to keep merging, because it means >>>> the two processes are working in lockstep; it is not sufficient that >>>> the requests are just nearby, as for rotational disks. >>>> >>> >>> For jeff, at least "dump" utility threads were kind of working in lockstep >>> for writes and he gained significantly by merging these queues together. >> >> Actually, it was for reads. >> >>> So the argument is that CPU overhead saving in this case is more substantial >>> as compared to gains made by lockstep read threads. I think we shall have to >>> have some numbers to justify that. >> >> Agreed. Corrado, I know you don't have the hardware, so I'll give this >> a run through the read-test2 program and see if it regresses at all. > Great.
I ran the test program 50 times, and here are the results:
==> vanilla <== Mean: 163.22728 Population Std. Dev.: 0.55401
==> patched <== Mean: 162.91558 Population Std. Dev.: 1.08612
This looks acceptable to me.
Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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