From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: random_generator=lfsr overhead with more disks? References: <9FD577EB-5F83-4AB3-9FBA-704094437C15@gmail.com> From: Jens Axboe Message-ID: <512d2421-2399-07db-9ddf-b6776614bae4@kernel.dk> Date: Wed, 21 Mar 2018 08:34:56 -0600 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit To: Jeff Furlong , Michael Green , "fio@vger.kernel.org" List-ID: On 3/16/18 12:45 PM, Jeff Furlong wrote: > A few comments: > > -How does the cpu utilization look compared between the lfsr and > default tausworthe generators? Is the cpu util actually higher on > lfsr? If it is near saturated on 16 devices, then naturally > throughput would decrease. The LFSR generator is constant overhead, and it should not degrade as more jobs are using it. There's no shared state at all. If I were to guess, then the identical offsets of the jobs is probably a likely clue, since that is the only real difference between the two random generators. Are these 16 volumes all completely separate? Finally, you are having 8 jobs all accessing the same 16 devices. You'll likely get more consistent performance if you split things up a bit by having 4 jobs, each doing IO to just two of the disks. This shouldn't really be impacted by the random generator, but your current setup is prone to performance fluctuations. -- Jens Axboe