Re: [User-mode-linux-devel] Performance enhancement idea...

[Jose R. <re...@cs...>]

On Fri, 7 Jul 2000, Chris Lattner wrote:

> The only problem is that you only have two bytes of code space to work
> with (int 0x80 is encoded as 0xCD 0x80), and a call takes 5? bytes...
> 
> :(  If there is any other useful thing to fit into two bytes... that would
> be cool too!  

 There are some tools for instrumenting code.

http://www.cs.umd.edu/projects/dyninstAPI/

 A small modification would make possible to change "int $80" for more
elavorate code. This has to be done when the code is loaded
(fs/binfmt_elf.c) but I think that it's feasable.

> On Fri, 7 Jul 2000, Jose Renau wrote:
> 
> > 
> >  If the idea is to increase the performance, why not no change the "int
> > $80" for a "call um_syscall". um_syscall can "communicate" when it's
> > strictly necessary. For example, a getpid could be done locally.
> > 
> >  This optimization could do te umlinux even faster than the "default"
> > Linux.
> > 
> >  Suggestions?
> > 
> > On Fri, 7 Jul 2000, Chris Lattner wrote:
> > 
> > > Currently, syscall entry requires the tracing thread to wake up, munge the
> > > registers around, and do other stuff before the syscall'ing thread
> > > continues in the kernel...  I have two ideas to improve syscall
> > > performance:
> > > 
> > > 1. Instead of changing EAX to make the syscall do a getpid, why not just
> > > add 2 to the EIP to skip over the int 80?
> > > 2. Additionally, why not change that int 80 to be "ud2" or some other
> > > opcode that will cause a SIGILL?  When receiving a sigill, just check for
> > > the magic opcode and if it is set, do the syscall... this would have the
> > > advantage that the ptrace/c-switch overhead would be reduced...
> > > 
> > > Does anyone see any problems with these suggestions?  I may be interested
> > > in implementing them if not...
> > > 
> > > -Chris
> > > 
> > > 
> > > 
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> > > 
> > 
> > -------------------------------------------------------------------
> >  Jose Renau Ardevol        | The great things in life are what they
> >             re...@ui... |  seem to be.           Oscar Wilde
> > -------------------------------------------------------------------
> > 
> > 
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> > 
> 

-------------------------------------------------------------------
 Jose Renau Ardevol        | The great things in life are what they
            re...@ui... |  seem to be.           Oscar Wilde
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