Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!feeder.news-service.com!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Robert Klemme Newsgroups: comp.lang.java.programmer Subject: Re: Bulk Array Element Allocation, is it faster? Date: Sun, 25 Sep 2011 12:57:56 +0200 Lines: 67 Message-ID: <9e8fplF19bU1@mid.individual.net> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: individual.net OFeomkXuNFfgL+ip57HEXgHyn2iA9roDulsybikhz87cJl/Bg= Cancel-Lock: sha1:8HMuiCDCFMT7E7XBS0J4a14KsRM= User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.21) Gecko/20110831 Lightning/1.0b2 Thunderbird/3.1.13 In-Reply-To: Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:8288 On 09/25/2011 03:41 AM, Patricia Shanahan wrote: > On 9/24/2011 6:17 PM, Jan Burse wrote: >> Dear All, >> >> I just wonder wether modern JIT do optimize >> Code of the following form: >> >> Bla[] bla = new Bla[n]; >> for (int i=0; i> bla[i] = new Bla(); >> } >> >> When I use the above in my code, my application >> spends 8800 ms in the benchmark I have. >> >> When I then change the code to: >> >> Bla[] bla = new Bla[n]; >> >> ... >> >> if (bla[i]==null) >> bla[i] = new Bla(); >> >> .. >> >> So instead of allocating all elements at once, >> I will allocate them on demand in the subsequent >> flow of my application. >> >> When I use the above in my code, my application >> now suddently sends 9600 ms in the benchmark >> I have. >> >> So I wonder whether eventually the JIT has >> a way to detect the bulk allocate of the first >> version and transform it into something more >> efficient than my lazy allocation. >> >> Any clues? > > You also need to consider the general optimization of processors in > favor of doing efficiently those things they have done in the recent past. > > When you do the allocation all at once, the code and data for "new > Bla()" is in cache on the second and subsequent calls. There may be > cache conflicts between "new Bla()" and the actual work, leading to > many more cache misses when you interleave them. > > Doing the initialization on demand may be adding an unpredictable > conditional branch to the subsequent flow. This and the fact that lazy initialization has concurrency issues when used in a multi threading context (which e.g. final members do not have) has made me use this approach less frequently. Also, for short lived objects in total it might be much more efficient to just allocate the object even if it is not used because it won't survive new generation anyway. I think the lazy init idiom only really pays off if construction is expensive (e.g. because it involves IO or time consuming calculations). In all other cases it's better to just unconditionally create and let GC work. And because of improvements in JVM technology the balance has moved a lot away from the lazy side because allocation and deallocation overhead became smaller than in earlier Java versions. Kind regards robert