Messages in this thread | | | Date | Fri, 25 Jun 2010 09:51:37 -0500 | From | Nathan Fontenot <> | Subject | Re: [PATCH] memory hotplug disable boot option |
| |
On 06/25/2010 04:19 AM, Andi Kleen wrote: > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> writes: > >>> Proposed patch to disable memory hotplug via a boot option, >>> mem_hotplug=[on|off]. The patch only disables memory hotplug in that it >>> prevents the creation of the memory sysfs directories for memory sections. >>> >>> This patch is meant to help alleviate very long boot times on systems with >>> large memory (1+ TB) and many memory sections (10's of thousands). >> >> Why making simple /sys file is so slowly? Couldn't we fix such performance >> problem?
The issue is the large number of sysfs memory directories that get created. On a system with 1 TB of memory I am seeing ~63,00 directories. The long creation time is due to the string compare check in sysfs code to ensure we are not creating a directory with a duplicate name.
> > Yes I agree this really needs to be fixed properly, not hacked around > with an option. Nathan can you please post some profile logs of the long > boot times? >
At this point the only profiling data I have is from adding a printk before and after the creation of the memory sysfs files in drivers/base/memory.c and booting with printk.time=1.
With 250 GB of memory: 10 seconds [ 0.539562] Memory Start [ 10.450409] Memory End
With 1 TB of memory: 9.1 minutes [ 31.680168] Memory Start [ 584.186500] Memory End
I am hoping to get access to a machine with 2 TB of memory sometime soon and can post data for boot times on an unpatched kernel.
I posted a patch earlier that updated sysfs to use reb-black trees to store the sysfs dirent structs (though the patch has issues with namespaces). Using this patch on a 1 TB system the memory sysfs dir creation dropped to 2.2 minutes.
[ 1.295874] Memory Start [ 137.293510] Memory End
On a system with 2 TB of memory, the reb-black tree patch dropped the sysfs file creation time down to 33 minutes and total boot time to 1 hour 15 minutes. I haven't measured an unpatched kernel, but total boot of an unpatched kernel is just over 8 hours.
With 2 TB and patched kernel: 33 minutes [ 3.241679] Memory Start [ 1986.973324] Memory End
I am open to other ideas on a solution for this. With this many memory sections I feel that the sysfs memory directory really isn't human readable, not with 10's or 100's of thousands of entries. Perhaps moving to a flat file representation and not create all the directories?
-Nathan
| |