Allocating memory beyond mem_limit if p->slabs <= 0

[Chris Eberly]

Yeah I looked over the code, I'm pretty sure you're right. I kind of
forget my original rationale for posting this, I think we were trying
to figure out why we were able to allocate large objects after we had
thought to have already filled up the cache. Anyway I don't have
access to enough historical information to make any valid points :)
Thanks for responding, though.

Chris Eberly

On Nov 14, 2007 9:39 AM, Chris Eberly <chris.eberly at gmail.com> wrote:
> ( No one has responded :) )
>
> I agree with the first part of your response, it does allow you to
> allocate a slab of every size regardless of the malloc limit. I might
> be misunderstanding the code as well, but in this function we are
> already past the point of checking the LRU, we are simply deciding
> whether we can allocate a new slab or not. Anyhow, the first statement
> is a good rationale for this code, and it hardly matters for the large
> memory installations most people seem to be using. I'm new to the
> source code so I'm just double checking my assumptions about the
> design decisions. Thanks!
>
> Chris Eberly
>
>
> On Nov 14, 2007 12:06 AM, dormando <dormando at rydia.net> wrote:
> > (Has anyone responded to this?)
> >
> > It's my understanding (although I could be wrong, this code's just a little
> > dicey), that if you're not pre-allocating your slabs, that fix prevented
> > transient out of memory errors if you hit the malloc limit before having
> > one slab of every size available.
> >
> > Since that's counting the malloc'ed memory and not the "used" memory, you'd
> > otherwise end up stuck having OOM errors on every store of an item of
> > specific size, even if there's plenty of room in in the LRU. Which there
> > always is unless you run memcached with the -M option.
> >
> > -Dormando
> >
> >
> > Chris Eberly wrote:
> > > Hello,
> > > We are working on optimizing our memcached configuration and I came
> > > across something I don't understand. It seems as though you are able to
> > > allocate a slab (and therefore more memory) even if you have passed the
> > > command line memory limit with other slabs. The code in question is in
> > > slabs.c in do_slabs_newslab():
> > >
> > >     if (mem_limit && mem_malloced + len > mem_limit && p->slabs > 0)
> > >         return 0;
> > >
> > > So I can theoretically allocate 1 of each type of slab I don't have yet
> > > even if I am passed my memory limit. While it turns out this is
> > > advantageous (or at worst irrelevant because of the scale of the memory
> > > we are using), I am wondering what the rationale behind this code is?
> > >
> > >
> > > Chris Eberly
> >
> >
>