Date: Thu, 4 Oct 2007 21:02:53 +0200 (CEST) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-stable@FreeBSD.ORG, cb@severious.net, matrix@itlegion.ru Subject: Re: Quation about HZ kernel option Message-ID: <200710041902.l94J2rFV075995@lurza.secnetix.de> In-Reply-To: <20071004143944.GA46491@nowhere>
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Craig Boston wrote: > Oliver Fromme wrote: > > In that case, I would recommend not to override the > > default at all (which is 1000). > > ISTM that it would be better to use kern.hz=100 in this case. I haven't seen a benchmark yet which would support that. > My reasoning is that a web server shouldn't be terribly sensitive to > latency, so it's better to have longer quantums to get more work done > without context switching overhead. With HZ=1000 any modern CPU still performs millions of instructions per scheduling quantum. The context switch overhead for the case that a process exceeds its time slice is negligible. > If you're not using polling, you'll > be getting interrupts for network traffic anyway. Right. Probably many more of them than the scheduler switches between processes. > With polling on however, a high HZ value makes sense. Yes, I've seen servers running at HZ=5000 and even more. > > Basically, the kernel cannot handle time slices smaller > > than 1/HZ seconds, for any purpose. > > It should still be able to schedule a new process for the remainder of > the slice should the current one block or yield though, right? Right. And in that case a context switch happens anyway, so reducing the HZ value doesn't buy you anything, except reducing latency for those processes that can do their job quickly (e.g. serving static inline images). I still believe that it's best to not modify the default value of HZ=1000 on such a server. Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd "To this day, many C programmers believe that 'strong typing' just means pounding extra hard on the keyboard." -- Peter van der Linden
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