From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id A1A6A6B0033 for ; Wed, 11 Jan 2017 20:47:56 -0500 (EST) Received: by mail-pf0-f200.google.com with SMTP id z128so15833342pfb.4 for ; Wed, 11 Jan 2017 17:47:56 -0800 (PST) Received: from mga07.intel.com (mga07.intel.com. [134.134.136.100]) by mx.google.com with ESMTPS id d2si7517629pli.105.2017.01.11.17.47.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 11 Jan 2017 17:47:55 -0800 (PST) From: "Huang\, Ying" Subject: Re: [PATCH v5 2/9] mm/swap: Add cluster lock References: <20170111150029.29e942aa00af69f9c3c4e9b1@linux-foundation.org> <20170111160729.23e06078@lwn.net> <20170111151526.e905b91d6f1ee9f21e6907be@linux-foundation.org> Date: Thu, 12 Jan 2017 09:47:51 +0800 In-Reply-To: <20170111151526.e905b91d6f1ee9f21e6907be@linux-foundation.org> (Andrew Morton's message of "Wed, 11 Jan 2017 15:15:26 -0800") Message-ID: <8760ll122g.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Jonathan Corbet , Tim Chen , "Huang, Ying" , dave.hansen@intel.com, ak@linux.intel.com, aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Shaohua Li , Minchan Kim , Rik van Riel , Andrea Arcangeli , "Kirill A . Shutemov" , Vladimir Davydov , Johannes Weiner , Michal Hocko , Hillf Danton , Christian Borntraeger Hi, Andrew, Andrew Morton writes: > On Wed, 11 Jan 2017 16:07:29 -0700 Jonathan Corbet wrote: > >> On Wed, 11 Jan 2017 15:00:29 -0800 >> Andrew Morton wrote: >> >> > hm, bit_spin_lock() is a nasty thing. It is slow and it doesn't have >> > all the lockdep support. >> > >> > Would the world end if we added a spinlock to swap_cluster_info? >> >> FWIW, I asked the same question in December, this is what I got: >> >> ... >> >> > > Why the roll-your-own locking and data structures here? To my naive >> > > understanding, it seems like you could do something like: >> > > >> > > struct swap_cluster_info { >> > > spinlock_t lock; >> > > atomic_t count; >> > > unsigned int flags; >> > > }; >> > > >> > > Then you could use proper spinlock operations which, among other things, >> > > would make the realtime folks happier. That might well help with the >> > > cache-line sharing issues as well. Some of the count manipulations could >> > > perhaps be done without the lock entirely; similarly, atomic bitops might >> > > save you the locking for some of the flag tweaks - though I'd have to look >> > > more closely to be really sure of that. >> > > >> > > The cost, of course, is the growth of this structure, but you've already >> > > noted that the overhead isn't all that high; seems like it could be worth >> > > it. >> > >> > Yes. The data structure you proposed is much easier to be used than the >> > current one. The main concern is the RAM usage. The size of the data >> > structure you proposed is about 80 bytes, while that of the current one >> > is about 8 bytes. There will be one struct swap_cluster_info for every >> > 1MB swap space, so for 1TB swap space, the total size will be 80M >> > compared with 8M of current implementation. > > Where did this 80 bytes come from? That swap_cluster_info is 12 bytes > and could perhaps be squeezed into 8 bytes if we can get away with a > 24-bit "count". Sorry, I made a mistake when measuring the size of swap_cluster_info when I sent that email, because I turned on the lockdep when measuring. I have sent out a correction email to Jonathan when I realized that later. So the latest size measuring result is: If we use bit_spin_lock, the size of cluster_swap_info will, - increased from 4 bytes to 8 bytes on 64 bit platform - keep as 4 bytes on 32 bit platform If we use normal spinlock (queue spinlock), the size of cluster_swap_info will, - increased from 4 bytes to 8 bytes on 64 bit platform - increased from 4 bytes to 8 bytes on 32 bit platform So the difference occurs on 32 bit platform. If the size increment on 32 bit platform is OK, then I think it should be good to use normal spinlock instead of bit_spin_lock. Personally, I am OK for that. But I don't know whether there will be some embedded world people don't like it. Best Regards, Huang, Ying -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org