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    SubjectRe: [RFC PATCH] LTTng instrumentation mm (updated)
    * Dave Hansen (haveblue@us.ibm.com) wrote:
    > On Fri, 2007-11-30 at 12:05 -0500, Mathieu Desnoyers wrote:
    > >
    > >
    > > Given a trace including :
    > > - Swapfiles initially used
    > > - multiple swapon/swapoff
    > > - swap in/out events
    > >
    > > We would like to be able to tell which swap file the information has
    > > been written to/read from at any given time during the trace.
    >
    > Oh, tracing is expected to be on at all times? I figured someone would
    > encounter a problem, then turn it on to dig down a little deeper, then
    > turn it off.
    >

    Yep, it can be expected to be on at all times, especially on production
    systems using "flight recorder" tracing to record information in a
    circular buffer, then dumping the buffers when some triggers (error
    conditions) happens.

    > As for why I care what is in /proc/swaps. Take a look at this:
    >
    > struct swap_info_struct *
    > get_swap_info_struct(unsigned type)
    > {
    > return &swap_info[type];
    > }
    >
    > Then, look at the proc functions:
    >
    > static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
    > {
    > struct swap_info_struct *ptr;
    > struct swap_info_struct *endptr = swap_info + nr_swapfiles;
    >
    > if (v == SEQ_START_TOKEN)
    > ptr = swap_info;
    > ...
    >
    > I guess if that swap_info[] has any holes, we can't relate indexes in
    > there right back to /proc/swaps, but maybe we should add some
    > information so that we _can_.
    >

    The if (!(ptr->flags & SWP_USED) test in swap_next seems to skip the
    unused swap_info entries.

    Why should we care about get_swap_info_struct always returning a "used"
    swap info struct ?

    > -- Dave
    >

    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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