Re: [RFC] Peeling off dcache_lock

[Dipankar Sarma]

Since there has been speculation about the throughput #s for lower end,
I have put up the comparison graph in the same page 
(http://lse.sf.net/locking/dcache/dcache_lock.html).

As http://lse.sourceforge.net/locking/dcache/results/dbench/base+dc-tpt.png
would show, there is *no* performance degradation at the lower end or
UP. The absolute numbers are in
http://lse.sourceforge.net/locking/dcache/results/dbench/.

Thanks
Dipankar
-- 
Dipankar Sarma  <[e-mail]> http://lse.sourceforge.net
Linux Technology Center, IBM Software Lab, Bangalore, India.

On Mon, Jan 21, 2002 at 05:40:39PM +0530, Maneesh Soni wrote:
> Hi All,
> 
> We have been doing experiments with dcache_lock to provide some relief from it.
> Though dcache_lock is not a very hot lock in comparision to BKL but on higher
> end machines it becomes quite contentious. We would like to have feedbacks,
> comments about the approach taken and guidance on how to improve this further.
> 
> As dcache_lock is acquired maximum number of times while doing lookup, the main
> idea was to remove dcache_lock from d_lookup. With the help of Read Copy Update
> it is possible to lookup the hash list of dentries without taking dcache_lock
> but as d_lookup also updates the lru list, removing dcache_lock from d_lookup
> was not straight forward. Various approaches were tried for that and the most
> successful is doing lazy updation of lru list. 
> 
> Using this the dcache_lock contention was down from 16.5% to 0.95% on an 
> 8-way SMP box and lock utilization was down from 5.3% to 0.89%
> 
> The patch for lazy lru updation using RCU can be found here:
> 
> http://lse.sourceforge.net/locking/dcache/patches/2.4.17/dcache_rcu-lazy_lru-2.4.17-06.patch
> 
> The basic conecpt for this approach is to not update the lru list in d_lookup
> facilitating lockless d_lookup. The lru list can now have dentries with non zero
> reference count also. The lru list is updated while pruning dentry cache. The
> dcache_lock is kept around so that there are no changes in the update side 
> locking.
> 
> The implementation details for lazy lru list updation and other approaches
> can be found here:
> 
> http://lse.sourceforge.net/locking/dcache/dcache_lock.html 
> 
> The above patch works fine with various file systems like ext2, JFS, ext3, /procand has been tested ok with ltp-20020108, dbench, httperf. And doesnot not have
> any adverse effects on uni processor performance.
> 
> 
> Regards,
> Maneesh
> 
> 
> -- 
> Maneesh Soni
> IBM Linux Technology Center, 
> IBM India Software Lab, Bangalore.
> Phone: [phone] email: [e-mail]
> http://lse.sourceforge.net/locking/rcupdate.html