Date: Wed, 6 Jun 2007 02:30:00 +0400 From: Solar Designer <solar@...nwall.com> To: john-users@...ts.openwall.com Subject: "Idle" setting: system responsiveness and performance On Tue, Jun 05, 2007 at 10:12:16AM +0100, Evo Eftimov, iSec Consulting, www.isecc.com wrote: > If Idle=N will the user interface of the system become unresponsive No, but it will feel slower. I recommend "Idle=Y" for almost all uses of John, with the only exception being benchmarks. There's almost no difference in performance of John itself between these two settings when running on a system with no similar load. Server-type load is OK, and "Idle=Y" helps reduce (in fact, almost eliminate) the impact John would otherwise have on server response times. The same applies to uses of John on a PC or workstation. I'll try to explain: When a request arrives to a server (e.g., for a web page) or you do typical work on a PC or workstation (e.g., start a GUI application that needs to display its window), processing of such a request (received from the remote client or initiated by you) will take a certain amount of processor time. That amount of processor time is almost constant (for a given computer), and, with no idle CPU available, it will be taken away from John regardless of the "Idle" setting. With "Idle=Y" and no idle CPU available, John will pause its work while the interactive request is being handled - so the request is handled quicker (in terms of real time). With "Idle=N" and no idle CPU available, John will compete for CPU time with handling of the interactive request - resulting in the request taking longer to be handled (again, in terms of real time). Yes, John will "win" some CPU cycles for itself during that time - but by doing so it will increase the duration of this "fight". So in the long run it will not advance any further than it would if it paused gracefully. When there is in fact an idle CPU available (on a multi-CPU system), the "Idle" setting makes no difference. One thing I did not mention in the simplified description above (but I do mention it briefly now) is that the "fight" for CPU time often has an impact on the efficiency of caches. So the lack of a "fight", when John is configured with "Idle=Y" and thus it waits for the request to be handled, might actually result in slight performance increase for all tasks that would otherwise compete for CPU time (and cache space). When there's other load that is in fact similar to John's and the number of CPU cores is smaller than the number of threads that cause such load, things are different. With "Idle=N" and no special scheduling priority settings for any of the processes or threads, each thread is likely to get an equal share of CPU time. With "Idle=Y" and no other special scheduling priority settings for any of the other processes or threads, John will almost stop (and stay that way) and let the other tasks run, which is probably not what you want. On systems with SMT (simultaneous multithreading) processors, with Intel Pentium 4 with Hyperthreading and Sun UltraSPARC T1 ("Niagara") being two examples, the effect of "Idle" is less obvious and more OS-specific. -- Alexander Peslyak <solar at openwall.com> GPG key ID: 5B341F15 fp: B3FB 63F4 D7A3 BCCC 6F6E FC55 A2FC 027C 5B34 1F15 http://www.openwall.com - bringing security into open computing environments -- To unsubscribe, e-mail john-users-unsubscribe@...ts.openwall.com and reply to the automated confirmation request that will be sent to you.
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