lkml.org 
[lkml]   [2008]   [Aug]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC:Patch: 000/008](memory hotplug) rough idea of pgdat removing
> Yasunori Goto wrote:
>
> > Current my idea is using RCU feature for waiting them.
> > Because it is the least impact against reader's performance,
> > and pgdat remover can wait finish of reader's access to pgdat
> > which is removing by synchronize_sched().
>
> The use of RCU disables preemption which has implications as to
> what can be done in a loop over nodes or zones.

Yeap. It's the one of (big) cons.

> This would also potentially add more overhead to the page allocator hotpaths.

Agree.

To tell the truth, I tried hackbench with 3rd patch which add rcu_read_lock
in hot-path before this post to make rough estimate its impact.

%hackbench 100 process 2000

without patch.
39.93

with patch
39.99
(Both is 10 times avarage)

I guess this result has effect of disable preemption.
So, throughput looks not so bad, but probably, latency would be worse
as you mind.

Kame-san advised me I should take more other benchmarks which can get memory
performance. I'll do it next week.

> > If you have better idea, please let me know.
>
> Use stop_machine()? The removal of a zone or node is a pretty rare event
> after all and it would avoid having to deal with rcu etc etc.
>

I thought it at first, but are there the following worst case?


CPU 0 CPU 1
-------------------------------------------------------
__alloc_pages()

parsing_zonelist()
:
enter page_reclarim()
sleep (and remember zone) :
:
update zonelist and node_online_map
with stop_machine_run()
free pgdat().
remove the Node electrically.

wake up and touch remembered
zone, but it is removed
(Oops!!!)



Anyway, I'm happy if there is better way than my poor idea. :-)

Thanks for your comment.


--
Yasunori Goto




\
 
 \ /
  Last update: 2008-08-01 11:59    [W:1.669 / U:0.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site