lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Wed, 24 Sep 2008 10:54:37 -0700 From: "Martin Bligh" <mbligh@...gle.com> To: "Linus Torvalds" <torvalds@...ux-foundation.org> Cc: "Peter Zijlstra" <peterz@...radead.org>, "Steven Rostedt" <rostedt@...dmis.org>, linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...e.hu>, "Thomas Gleixner" <tglx@...utronix.de>, "Andrew Morton" <akpm@...ux-foundation.org>, prasad@...ux.vnet.ibm.com, "Mathieu Desnoyers" <compudj@...stal.dyndns.org>, "Frank Ch. Eigler" <fche@...hat.com>, "David Wilder" <dwilder@...ibm.com>, hch@....de, "Tom Zanussi" <zanussi@...cast.net>, "Steven Rostedt" <srostedt@...hat.com> Subject: Re: [RFC PATCH 1/3] Unified trace buffer > Why "tsc_shifted"? > > I think 27 bits is probably fine, but not by removing precision. Instead > of shifting it so it will fit (and dropping low bits as uninteresting), do > it by encoding it as a delta against the previous thing. 27 bits would > still be sufficient for any high-performance thing that has tons and tons > of packets, and if you only have a few trace events you can afford to have > the "TSC overflow" event type (and if you want it that dense, you could > just make 'data' be the high bits, for a total of 59 bits rather than 64 > bits of TSC. > > 59 bits of cycle counters is perfectly fine unless you are talking trace > events over a year or so (I didn't do the math, but let's assume a 4GHz > TSC as a reasonable thing even going forward - even _if_ CPU's get faster > than that, the TSC is unlikely to tick faster since it's just not worth it > from a power standpoint). > > Ok, I did the math. 1<<27 seconds (assuming the low 32 bits are just > fractions) is something like 4+ years. I _really_ don't think we need more > than that (or even close to that) in TSC timestamps for tracing within one > single buffer. Mmm. Either I'm confused, or we're talking about different things. If we just record the TSC unshifted, in 27 bits, at 4GHz, that gives us about 1/30 of a second? So we either shift, use > 27 bits, or record at least 30 events a second, none of which I like much ... -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists