From: Theodore Ts'o Subject: Re: After unlinking a large file on ext4, the process stalls for a long time Date: Thu, 17 Jul 2014 09:37:55 -0400 Message-ID: <20140717133755.GN1491@thunk.org> References: <53C687B1.30809@free.fr> <21446.38705.190786.631403@quad.stoffel.home> <53C6B38A.3000100@free.fr> <59C3F41A-6AFD-418E-BCE6-2361B8140D9A@dilger.ca> <53C7A5CA.4050903@free.fr> <53C7B0B7.9030007@free.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: =?utf-8?B?THVrw6HFoQ==?= Czerner , Andreas Dilger , Ext4 Developers List , linux-fsdevel To: Mason Return-path: Content-Disposition: inline In-Reply-To: <53C7B0B7.9030007@free.fr> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Thu, Jul 17, 2014 at 01:17:11PM +0200, Mason wrote: > unlink("/mnt/hdd/xxx") = 0 <111.479283> > > 0.01user 111.48system 1:51.99elapsed 99%CPU (0avgtext+0avgdata 772maxresident)k > 0inputs+0outputs (0major+434minor)pagefaults 0swaps ... and we're CPU bound inside the kernel. Can you run perf so we can see exactly where we're spending the CPU? You're not using a journal, so I'm pretty sure what you will find is that we're spending all of our time in mb_free_blocks(), when it is updating the internal mballoc buddy bitmaps. With a journal, this work done by mb_free_blocks() is hidden in the kjournal thread, and happens after the commit is completed, so it won't block other file system operations (other than burning some extra CPU on one of the multiple cores available on a typical x86 CPU). Also, I suspect the CPU overhead is *much* less on an x86 CPU, which has native bit test/set/clear instructions, whereas the MIPS architecture was designed by Prof. Hennessy at Stanford, who was a doctrinaire RISC fanatic, so there would be no bitop instructions. Even though I'm pretty sure what we'll find, knowing exactly *where* in mb_free_blocks() or the function it calls would be helpful in knowing what we need to optimize. So if you could try using perf (assuming that the perf is supported MIPS; not sure if it does) that would be really helpful. Thanks, - Ted