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Date: Wed, 11 Jan 2017 17:58:12 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Huang\, Ying" <ying.huang@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
        Tim Chen <tim.c.chen@linux.intel.com>, <dave.hansen@intel.com>,
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        Hillf Danton <hillf.zj@alibaba-inc.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH v5 2/9] mm/swap: Add cluster lock
Message-Id: <20170111175812.9e459e4c51502265aad5f2dc@linux-foundation.org>
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On Thu, 12 Jan 2017 09:47:51 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote:

> >> > 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.

I think that'll be OK - the difference is small and many small systems
disable swap anyway.  So can we please try that?  Please do describe
the additional overhead (with numbers) in the changelog: "additional
bytes of RAM per GB of swap", for example.  And please also rerun the
performance tests, see if we can notice the alleged speed improvements
from switching to a spinlock.