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Avoid negative values in malloc_utils metrics (#2692)

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## Proposed Changes

While investigating memory usage I noticed that the malloc metrics were going negative once they passed 2GiB. This is because the underlying `mallinfo` function returns a `i32`, and we were casting it straight to an `i64`, preserving the sign.

The long-term fix will be to move to `mallinfo2`, but it's still not yet widely available.
This commit is contained in:
Michael Sproul 2021-10-11 00:10:34 +00:00
parent 7c7ba770de
commit 229542cd6c
1 changed files with 21 additions and 11 deletions
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32
common/malloc_utils/src/glibc.rs
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@ -85,23 +85,33 @@ pub fn scrape_mallinfo_metrics() {
// The docs for this function say it is thread-unsafe since it may return inconsistent results.
// Since these are just metrics it's not a concern to us if they're sometimes inconsistent.
//
// There exists a `malloc2` function, however it was release in February 2021 and this seems too
// recent to rely on.
// There exists a `mallinfo2` function, however it was released in February 2021 and this seems
// too recent to rely on.
//
// Docs:
//
// https://man7.org/linux/man-pages/man3/mallinfo.3.html
let mallinfo = mallinfo();
set_gauge(&MALLINFO_ARENA, mallinfo.arena as i64);
set_gauge(&MALLINFO_ORDBLKS, mallinfo.ordblks as i64);
set_gauge(&MALLINFO_SMBLKS, mallinfo.smblks as i64);
set_gauge(&MALLINFO_HBLKS, mallinfo.hblks as i64);
set_gauge(&MALLINFO_HBLKHD, mallinfo.hblkhd as i64);
set_gauge(&MALLINFO_FSMBLKS, mallinfo.fsmblks as i64);
set_gauge(&MALLINFO_UORDBLKS, mallinfo.uordblks as i64);
set_gauge(&MALLINFO_FORDBLKS, mallinfo.fordblks as i64);
set_gauge(&MALLINFO_KEEPCOST, mallinfo.keepcost as i64);
/// Cast a C integer as returned by `mallinfo` to an unsigned i64.
///
/// A cast from `i32` to `i64` preserves the sign bit, resulting in incorrect negative values.
/// Going via `u32` treats the sign bit as part of the number.
///
/// Results are still wrong for memory usage over 4GiB due to limitations of mallinfo.
fn unsigned_i64(x: i32) -> i64 {
x as u32 as i64
}
set_gauge(&MALLINFO_ARENA, unsigned_i64(mallinfo.arena));
set_gauge(&MALLINFO_ORDBLKS, unsigned_i64(mallinfo.ordblks));
set_gauge(&MALLINFO_SMBLKS, unsigned_i64(mallinfo.smblks));
set_gauge(&MALLINFO_HBLKS, unsigned_i64(mallinfo.hblks));
set_gauge(&MALLINFO_HBLKHD, unsigned_i64(mallinfo.hblkhd));
set_gauge(&MALLINFO_FSMBLKS, unsigned_i64(mallinfo.fsmblks));
set_gauge(&MALLINFO_UORDBLKS, unsigned_i64(mallinfo.uordblks));
set_gauge(&MALLINFO_FORDBLKS, unsigned_i64(mallinfo.fordblks));
set_gauge(&MALLINFO_KEEPCOST, unsigned_i64(mallinfo.keepcost));
}
/// Perform all configuration routines.