Messages in this thread | | | Date | Tue, 21 Jun 2011 01:34:51 +0800 | From | Cong Wang <> | Subject | Re: [PATCH 1/3] mm: completely disable THP by transparent_hugepage=never |
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于 2011年06月21日 01:28, Rik van Riel 写道: > On 06/20/2011 01:19 PM, Cong Wang wrote: >> 于 2011年06月21日 01:10, Rik van Riel 写道: >>> On 06/20/2011 01:07 PM, Cong Wang wrote: >>>> 于 2011年06月21日 00:58, Mel Gorman 写道: >>>>> On Tue, Jun 21, 2011 at 12:34:28AM +0800, Amerigo Wang wrote: >>>>>> transparent_hugepage=never should mean to disable THP completely, >>>>>> otherwise we don't have a way to disable THP completely. >>>>>> The design is broken. >>>>>> >>>>> >>>>> I don't get why it's broken. Why would the user be prevented from >>>>> enabling it at runtime? >>>>> >>>> >>>> We need to a way to totally disable it, right? Otherwise, when I >>>> configure >>>> THP in .config, I always have THP initialized even when I pass "=never". >>>> >>>> For me, if you don't provide such way to disable it, it is not flexible. >>>> >>>> I meet this problem when I try to disable THP in kdump kernel, there is >>>> no user of THP in kdump kernel, THP is a waste for kdump kernel. This is >>>> why I need to find a way to totally disable it. >>> >>> What you have not explained yet is why having THP >>> halfway initialized (but not used, and without a >>> khugepaged thread) is a problem at all. >>> >>> Why is it a problem for you? >> >> It occupies some memory, memory is valuable in kdump kernel (usually >> only 128M). :) Since I am sure no one will use it, why do I still need >> to initialize it at all? > > Lets take a look at how much memory your patches end > up saving. > > By bailing out earlier in hugepage_init, you end up > saving 3 sysfs objects, one slab cache and a hash > table with 1024 pointers. That's a total of maybe > 10kB of memory on a 64 bit system. > > I'm not convinced that a 10kB memory reduction is > worth the price of never being able to enable > transparent hugepages when a system is booted with > THP disabled... >
Even if it is really 10K, why not save it since it doesn't much effort to make this. ;) Not only memory, but also time, this could also save a little time to initialize the kernel.
For me, the more serious thing is the logic, there is no way to totally disable it as long as I have THP in .config currently. This is why I said the design is broken.
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