linux-kernel - Re: [PATCH v4 0/3] ext4: increase mbcache scalability

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Message-Id: <DBA818DD-1C87-4B98-8475-8BB8F00F7DA4@dilger.ca>
Date:	Fri, 24 Jan 2014 23:09:24 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	T Makphaibulchoke <tmac@...com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"tytso@....edu" <tytso@....edu>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"aswin@...com" <aswin@...com>
Subject: Re: [PATCH v4 0/3] ext4: increase mbcache scalability

I think the ext4 block groups are locked with the blockgroup_lock that has about the same number of locks as the number of cores, with a max of 128, IIRC.  See blockgroup_lock.h. 

While there is some chance of contention, it is also unlikely that all of the cores are locking this area at the same time.  

Cheers, Andreas

> On Jan 24, 2014, at 14:38, Andi Kleen <andi@...stfloor.org> wrote:
> 
> T Makphaibulchoke <tmac@...com> writes:
> 
>> The patch consists of three parts.
>> 
>> The first part changes the implementation of both the block and hash chains of
>> an mb_cache from list_head to hlist_bl_head and also introduces new members,
>> including a spinlock to mb_cache_entry, as required by the second part.
> 
> spinlock per entry is usually overkill for larger hash tables.
> 
> Can you use a second smaller lock table that just has locks and is 
> indexed by a subset of the hash key. Most likely a very small 
> table is good enough.
> 
> Also I would be good to have some data on the additional memory consumption.
> 
> -Andi
> 
> -- 
> ak@...ux.intel.com -- Speaking for myself only
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