Messages in this thread | | | From | kamezawa.hiroyu@jp ... | Date | Sun, 7 Sep 2008 01:00:55 +0900 (JST) | Subject | Re: Re: [PATCH] [RESEND] x86_64: add memory hotremove config option |
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----- Original Message ----- >* Yasunori Goto <y-goto@jp.fujitsu.com> wrote: > >> I don't think its driver is almighty. IIRC, balloon driver can be >> cause of fragmentation for 24-7 system. >> >> In addition, I have heard that memory hotplug would be useful for >> reducing of power consumption of DIMM. >> >> I have to admit that memory hotplug has many issues, but I would like >> to solve them step by step. > >What would be nice is to insert the information both during bootup and >in /proc/meminfo and 'free' output that hot-removable memory segments >are not generic free memory, it's currently a limited resource that >might or might not be sufficient to serve a given workload. > >Perhaps even exclude it from 'total' memory reported by meminfo - to be >on the safe side of user expectations. In terms of user-space memory it >is already generic swappable memory but in terms of kernel-space >allocations it is not. > I wonder why anyone doesn't talk about ZONE_MOVABLE...When I wrote memory hotplug, I assumed help of ZONE_MOVABLE and SPARSEMEM. It is shown in meminfo.(I think memory hotplug is useful only when ZONE_MOVABLE is used.)
Most of problems which Goto wrote are mainly about placement of memmap and pgdat, zones. One example is that "when SPARSEMEM_VMEMMAP is enabled, memmap is not removed even when memory is removed. "
>As i said it earlier in the thread, i certainly have no objections from >the x86 maintenance side - nothing is worse than a generic kernel >feature only available on certain less frequently used platforms. Memory >hotplug has been available for some time in the MM and it's not really >causing any maintenance trouble at the moment and it is not enabled by >default either. > >Having said that, i have my doubts about its generic utility (the power >saving aspects are likely not realizable - nobody really wants DIMMs to >just sit there unused and the cost of dynamic migration is just >horrendous) - but as long as it's opt-in there's no reason to limit the >availability of an in-kernel feature artificially.
Nobody ? maybe just a trade-off problem in user side. Even without DIMM hotplug or DIMM's power save mode, making a DIMM idle is of no use ? I think memory consumes much power when it used. Memory Hotplug and ZONE_MOVABLE can make some memory idle. (I'm sorry if my thinking is wrong.)
> >Removing those limitations of kernel-space allocations should indeed be >done in baby steps - and whether it's worth turning such memory into >completely generic kernel memory is an open question. > I think generic kernel space memory hotplug will never be available.
>But the fact that a piece of memory is not fully generic is no reason >not to allow users to create special, capability-limited RAM resources >like they can already do via hugetlbfs or ramfs, as long as the the >capability limitations are advertised clearly. > Hmm, adding a feature like - offline some memory at boot. - online-memory-as-hugeltb mode is useful for generic pc users ?
Regards, -Kame
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