Messages in this thread | | | Date | Mon, 4 Jan 2010 06:20:31 +0530 | From | Balbir Singh <> | Subject | Re: [RFC] Shared page accounting for memory cgroup |
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* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-01-04 09:35:28]:
> On Mon, 4 Jan 2010 05:37:52 +0530 > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-01-04 08:51:08]: > > > > > On Tue, 29 Dec 2009 23:57:43 +0530 > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > > > > > Hi, Everyone, > > > > > > > > I've been working on heuristics for shared page accounting for the > > > > memory cgroup. I've tested the patches by creating multiple cgroups > > > > and running programs that share memory and observed the output. > > > > > > > > Comments? > > > > > > Hmm? Why we have to do this in the kernel ? > > > > > > > For several reasons that I can think of > > > > 1. With task migration changes coming in, getting consistent data free of races > > is going to be hard. > > Hmm, Let's see real-worlds's "ps" or "top" command. Even when there are no guarantee > of error range of data, it's still useful.
Yes, my concern is this
1. I iterate through tasks and calculate RSS 2. I look at memory.usage_in_bytes
If the time in user space between 1 and 2 is large I get very wrong results, specifically if the workload is changing its memory usage drastically.. no?
> > > 2. The cost of doing it in the kernel is not high, it does not impact > > the memcg runtime, it is a request-response sort of cost. > > > > 3. The cost in user space is going to be high and the implementation > > cumbersome to get right. > > > I don't like moving a cost in the userland to the kernel.
Me neither, but I don't think it is a fixed overhead.
Considering > real-time kernel or full-preemptive kernel, this very long read_lock() in the > kernel is not good, IMHO. (I think css_set_lock should be mutex/rw-sem...)
I agree, we should discuss converting the lock to a mutex or a semaphore, but there might be a good reason for keeping it as a spin_lock.
> cgroup_iter_xxx can block cgroup_post_fork() and this may cause critical > system delay of milli-seconds. >
Agreed, but then that can happen, even while attaching a task, seeing cgroup tasks file (list of tasks).
> BTW, if you really want to calculate somthing in atomic, I think following > interface may be welcomed for freezing. > > cgroup.lock > # echo 1 > /...../cgroup.lock > All task move, mkdir, rmdir to this cgroup will be blocked by mutex. > (But fork/exit will not be blocked.) > > # echo 0 > /...../cgroup.lock > Unlock. > > # cat /...../cgroup.lock > show lock status and lock history (for debug). > > Maybe good for some kinds of middleware. > But this may be difficult if we have to consider hierarchy. >
I don't like the idea of providing an interface that can control kernel locks from user space, user space can tangle up and get it wrong.
-- Balbir
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