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    SubjectRe: [Perfctr-devel] Re: [perfmon] perfmon2 code review: 32-bit ABI on 64-bit OS
    On Sat, Feb 11, 2006 at 02:33:54PM -0800, Stephane Eranian wrote:
    > Hello,
    > On Sun, Feb 12, 2006 at 12:16:25AM +0600, Philip Mucci wrote:
    > > >
    > > > On some 64-bit arches (e.g. x86_64), most userspace code is 64-bit,
    > > > while on others (e.g. powerpc), most is 32-bit. Reducing the number of
    > > > things that a userspace tool or library writer can trip over seems like
    > > > a good thing here, even if it slightly complicates perfmon's internals.
    > > >
    > > > > Note that there are similar issues with the remapped sampling buffer.
    > > > > There, you need to explicitly compile your tool with a special option
    > > > > to force certain types to be 64-bit (size_t, void *).
    > > >
    > > > It's pretty normal to just use 64-bit quantities in these cases, and
    > > > cast appropriately.
    > >
    > > I agree with Bryan. Stephane, do you have any quantitative data for how
    > > much more expensive going to 64 bit quantities would be? Which
    > > performance critical operations access this structure? AFAIK, any
    > > performance monitoring system call is already slow by nature...and thus
    > > an additional dozen cycles isn't going to make a difference. Of course,
    > > if this structure needs to be read/written by get_pmd, including the
    > > userspace version (+ mmap offset), then the extra overhead should be
    > > considered.
    > >
    > I think I can easily convert the bitmasks to be u64 on all platforms.
    > I don't think it will negatively impact performance on 32-bit applications.
    > The sampling buffer is another matter. It is directly remapped. The default
    > format, exposes size_t and void *. The size_t is not on the critical
    > path, it is used to specify the buffer size. If we expose as 64-bit,
    > we need to check on 32-bit system that the value is below 4GB and cast
    > to size_t.
    > The most challenging piece is the IP (program pointer) that is in every
    > sample. Today it is defined as unsigned long because this is fairly
    > natural for a code address. The 64bit OS captures addresses as 64-bit,
    > the 32-bit monitoring tool running on top has to consume them as 64-bit
    > addresses, so u64 would be fine.
    > But not on a 32-bit kernel with a 32-bit tool, addresses exported as u64
    > would certainly work but consume double to buffer space, and that is a
    > more serious issue in my mind.

    Hmm.. does the sampling buffer collect on userspace PC values, or
    kernel ones as well?

    David Gibson | I'll have my music baroque, and my code
    david AT | minimalist, thank you. NOT _the_ _other_
    | _way_ _around_!
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