Messages in this thread | | | Date | Thu, 21 Aug 2008 16:43:54 +0530 | From | Balbir Singh <> | Subject | Re: [discuss] memrlimit - potential applications that can use |
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KAMEZAWA Hiroyuki wrote: > On Thu, 21 Aug 2008 15:56:41 +0530 > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > >> KAMEZAWA Hiroyuki wrote: >>> On Thu, 21 Aug 2008 08:55:52 +0530 >>> Balbir Singh <balbir@linux.vnet.ibm.com> wrote: >>> >>>>>>> So, before we expand the use of those features to control groups by >>>>>>> adding a bunch of new code, let's make sure that there will be users >>>>>> for >>>>>>> it and that those users have no better way of doing it. >>>>>> I am all ears to better ways of doing it. Are you suggesting that overcommit was >>>>>> added even though we don't actually need it? >>>>> It serves a purpose, certainly. We have have better ways of doing it >>>>> now, though. "So, before we expand the use of those features to >>>>> control groups by adding a bunch of new code, let's make sure that there >>>>> will be users for it and that those users have no better way of doing >>>>> it." >>>>> >>>>> The one concrete user that's been offered so far is postgres. I've >>>> No, you've been offered several, including php and apache that use memory limits. >>>> >>>>> suggested something that I hope will be more effective than enforcing >>>>> overcommit. >>> I'm sorry I miss the point. My concern on memrlimit (for overcommiting) is that >>> it's not fair because an application which get -ENOMEM at mmap() is just someone >>> unlucky. >> It can happen today with overcommit turned on. Why is it unlucky? >> > Today's overcommit is also unlucky ;) > > For example) process A and B is under a memrlimit. > process A no memory leak, it often calls malloc() and free(). > process B does memory leak, 100MB per night. > > process A cannot do anything when it notices malloc() returns NULL. > It controls his memory usage perfectly. He is unlucky and will die. > process B can use up VSZ which is freed by process A. >
Yes, true that will happen. Why will A die because it sees NULL? Yes, many applications do die, but that is not how malloc == NULL is expected to be handled. If that is a concern, do not use any memrlimits for A and B, if you do you will find the bug early.
Now consider the other scenario, if there really is a memory leak and process B is using all that memory, two things to consider
1. Without swap controller, B will start swapping out A's memory and cause excessive swapping and performance loss 2. With swap controller enabled, at some point we will hit the swap limit, what happens then?
> (OOM-killer, is disliked by everyone, have some kind of fairness. > It checks usage.) > >> I think it's better to trigger some notifier to application or daemon >>> rather than return -ENOMEM at mmap(). Notification like "Oh, it seems the VSZ >>> of total application exceeds the limit you set. Although you can continue your >>> operation, it's recommended that you should fix up the situation". >>> will be good. >>> >> So you are suggesting that when we are running out of memory (as defined by our >> current resource constraints), we don't return -ENOMEM, but instead we now >> handle a new event that states that we are running out of memory? >> > Not "running out of memory" Just "VSZ is over the limit you set/expected". > > My point is an application witch can handle NULL returned by malloc() is > not very popular, I think. >
Yes and that's why we have the flexibility, if the application can't deal with it don't set memrlimits for those applications :)
-- Balbir
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