Messages in this thread | | | Date | Fri, 21 Mar 2008 18:26:44 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH prototype] [0/8] Predictive bitmaps for ELF executables |
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On Fri, Mar 21, 2008 at 10:15:15AM -0700, Ulrich Drepper wrote: > On Thu, Mar 20, 2008 at 2:00 AM, Andi Kleen <andi@firstfloor.org> wrote: > > What chaos exactly? For me it looks rather that a separatate database > > would be a recipe for chaos. e.g. for example how would you make sure > > the database keeps track of changing executables? > > I didn't say that a separate file with the data is better. In fact, I > agree, it's not much better. What I referred to as the problem is > that this is an extension which is not part of the ELF spec and
Linux executables already contain plenty of extensions outside the ELF spec like GNU_EH_FRAME or debuglink etc. It is not surprising because the ELF spec is kind of not maintained anymore afaik.
> doesn't fit in. The ELF spec has rules how programs have to deal with > unknown parts of a binary. Only this way programs like strip can work
Can you expand how the bitmap headers or pbitmap.c violate these rules?
> in the presence of extensions. There are ways to embed such a bitmap > but not ad-hoc as you proposed.
Concrete suggestions please.
> > > > But if the binutils leanred about this and added a bitmap phdr (it > > tends to be only a few hundred bytes even on very large executables) > > one seek could be avoided. > > And that is only one advantage. Let's not go done the path of invalid > ELF files.
What is invalid?
> > > > > Furthermore, by adding all this data to the end of the file you'll > > > > We are talking about 32bytes for each MB worth of executable. > > You can hardly call that "all that data". > > This wasn't a comment about the size of the data but the type of data. > The end of a binary contains data which is not used at runtime. Now > you're mixing in data which is used.
Well there was no other choice I know of short of relinking. Or do you have a way to add a PHDR without relinking? I am aware the SHDR is a hack, I called it that myself. I just don't know of a better way.
If the pbitmaps were universally adopted the use of the SHDRs would be phased out quickly I expect because the bitmaps would be standard parts of all PHDRs, but short term not requiring relinking is a huge advantage.
> Again, you misunderstand. I'm not proposing to exclude pages which > are only used at startup time. I mean the data collection should stop > some time after a process was started to account for possibly quite > different code paths which are taken by different runs of the program. > I.e., don't record page faults for the entire runtime of the program > which, hopefully in most cases, will result in all the pages of a > program to be marked (unless you have a lot of dead code in the binary > and it's all located together).
When would that time be? I cannot think of a single heuristic that would work for both "/bin/true" and a OpenOffice start.
-Andi
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