Messages in this thread | | | Date | Mon, 06 Aug 2007 12:26:37 +0200 | From | dragoran <> | Subject | Re: allow non root users to set io priority "idle" ? |
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Andi Kleen wrote: > dragoran <drago01@gmail.com> writes: > > >> Its possible to set the io priority using tools like ionice and syscalls. >> But this works only as root. >> Why can't a non root user change the priority of his processes to idle? >> I understand that realtime priority requires root but why idle? >> For instance the beagle trys to set its priority to idle but fails >> because it does not run as root. >> Any reason for this that I have missed? I can't think of a case where >> setting the io priority to idle would have a negative impact for other >> users or the whole system. >> > > Very low priority can starve others when it holds some kernel resource > needed by another task. > > Consider three tasks: one very low priority, one high priority: Low > priority task holds some kernel resource, middle task eats as much CPU > as it gets; high priority task wants to get the resource. High > priority will need to wait for low running, which could take a long > time. With true SCHED_IDLE (i believe the current implementation is > not true) this could be never or at least a very long time. > > There are ways to defend against this problem (known as priority inheritance), > but the kernel doesn't do them consistently. The same issue could also > happen for user space managed resources. > > For IO I suppose the same could happen too. e.g. low priority > task wants to write out a page and keeps it locked until the IO > is finished. High priority task wants to access the page and has > to wait until it is unlocked. Middle task generates an endless > stream of IO that makes the idle priority writeout never finish. couldn't this be fixed by bumping idle tasks to middle while they hold a pagelock? ex: task A - hp task B - mp task C - lp (idle) task C locks a page -> becomes a mp (middle prio) task until its finished task C unlocks the page -> back to idle
> In general idle priorities are quite risky, even for root. > > -Andi > >
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