Messages in this thread | | | Date | Tue, 31 Jan 2012 03:17:51 +0100 | Subject | Re: Bad SSD performance with recent kernels | From | Eric Dumazet <> |
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Le 30 janvier 2012 23:26, Vivek Goyal <vgoyal@redhat.com> a écrit : > > Can you please run the blktrace again with this patch applied. I am curious > to see how does traffic pattern look like now. > > In your previous trace, there were so many small 8 sector requests which > were merged into 512 sector requests before dispatching to disk. (I am > not sure why those requests are not bigger. Shouldn't readahead logic > submit a bigger request?) Now with plug/unplug logic removed, I am assuming > we should be doing less merging and dispatching more smaller requests. May be > that is helping and cutting down on disk idling time. > > In previous logs, 512 sector request seems to be taking around 1ms to > complete after dispatch. In between requests disk seems to be idle > for around .5 to .6 ms. Out of this .3 ms seems to be gone in just > coming up with new request after completion of previous one and another > .3ms seems to be consumed in merging the smaller IOs. So if we don't wait > for merging, it should keep disk busier for .3ms more which is 30% of time > it takes to complete 512 sector request. So theoritically it can give > 30% boost for this workload. (Assuming request size will not impact the > disk throughput very severely). > > Anyway, some blktrace data will shed some light.. > > Thanks > Vivek
I'll do that when I have access to the machine again, in 6 hours or so :)
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