From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Jiang Subject: Re: bcache compared to more RAM Date: Thu, 16 Aug 2012 09:20:48 -0700 Message-ID: <502D1DE0.8090300@intel.com> References: <201208150630.36861.csights@fastmail.fm> <502BCD0C.6080108@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-bcache-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Jack Wang Cc: John McCabe-Dansted , C Sights , linux-bcache-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, jack_wang-aA8Fybl81pAAvxtiuMwx3w@public.gmane.org, lindar_liu-aA8Fybl81pAAvxtiuMwx3w@public.gmane.org List-Id: linux-bcache@vger.kernel.org On Wed 15 Aug 2012 08:29:38 PM MST, Jack Wang wrote: > 2012/8/16 Dave Jiang : >> On 08/15/2012 04:51 AM, John McCabe-Dansted wrote: >>> On Wed, Aug 15, 2012 at 7:30 PM, C Sights wrote: >>>> Hi all, >>>> Has anyone tried creating a bcache device in RAM and benchmarking >>>> performance of that versus just allowing that RAM to be used as file system >>>> buffers? >>> I doubt it; bcache is designed to use non-volatile flash memory rather >>> than ram. >>> >>> If the filesystem is readonly, I imagine that using RAM for bcache >>> would lead to worse performance as it would just add overhead. If the >>> filesystem is read-write then it might lead to better performance but >>> would also lead to the file-system being trashed on if unexpected >>> power loss occurs. >>> >>> Under what circumstance would you intend to actually use ram backed bcache? >>> >> >> So the Intel Xeon E5-2600 series platforms has this feature called >> asynchronous DRAM refresh (ADR). Essentially it's battery backed DRAM on >> per DIMM socket basis. I have written a RAM disk driver on top of that >> experimentally and played a little bit with it used by bcache instead of >> SSD. Writing to the memory region isn't going to be as fast as normal >> cached RAM because you have to ensure the data is flushed to the DRAM. >> However, read should be the same as cached DRAM. But the writing should >> still be faster than SSD. Anyhow, I'm more curious as to how I can >> benchmark this to demonstrate if this is a benefit with bcache. i.e. >> what benchmarking tool(s) I should use and with what parameters etc. >> > Hi Dave, > > Intesting, wish to see the result. what cache mode is intended to use with ADR. > > Jack > -- Jack, Right now I'm using wb mode for cache. However when I write I call clflush_cache_range() and then read the last dword to make really sure that everything has made it to the DRAM. I do wish it's possible on IA to just flush without invalidate the cache though.