On Thu, Nov 16, 2017 at 08:25:58PM -0800, Khazhismel Kumykov wrote: > On Thu, Nov 16, 2017 at 8:50 AM, Shaohua Li <shli@xxxxxxxxxx> wrote: > > On Tue, Nov 14, 2017 at 03:10:22PM -0800, Khazhismel Kumykov wrote: > >> Allows configuration additional bytes or ios before a throttle is > >> triggered. > >> > >> This allows implementation of a bucket style rate-limit/throttle on a > >> block device. Previously, bursting to a device was limited to allowance > >> granted in a single throtl_slice (similar to a bucket with limit N and > >> refill rate N/slice). > >> > >> Additional parameters bytes/io_burst_conf defined for tg, which define a > >> number of bytes/ios that must be depleted before throttling happens. A > >> tg that does not deplete this allowance functions as though it has no > >> configured limits. tgs earn additional allowance at rate defined by > >> bps/iops for the tg. Once a tg has *_disp > *_burst_conf, throttling > >> kicks in. If a tg is idle for a while, it will again have some burst > >> allowance before it gets throttled again. > >> > >> slice_end for a tg is extended until io_disp/byte_disp would fall to 0, > >> when all "used" burst allowance would be earned back. trim_slice still > >> does progress slice_start as before and decrements *_disp as before, and > >> tgs continue to get bytes/ios in throtl_slice intervals. > > > > Can you describe why we need this? It would be great if you can describe the > > usage model and an example. Does this work for io.low/io.max or both? > > > > Thanks, > > Shaohua > > > > Use case that brought this up was configuring limits for a remote > shared device. Bursting beyond io.max is desired but only for so much > before the limit kicks in, afterwards with sustained usage throughput > is capped. (This proactively avoids remote-side limits). In that case > one would configure in a root container io.max + io.burst, and > configure low/other limits on descendants sharing the resource on the > same node. > > With this patch, so long as tg has not dispatched more than the burst, > no limit is applied at all by that tg, including limit imposed by > io.low in tg_iops_limit, etc. I'd appreciate if you can give more details about the 'why'. 'configuring limits for a remote shared device' doesn't justify the change.
