• Michael Anthony Knyszek's avatar
    runtime: don't coalesce scavenged spans with unscavenged spans · 6e9f664b
    Michael Anthony Knyszek authored
    As a result of changes earlier in Go 1.12, the scavenger became much
    more aggressive. In particular, when scavenged and unscavenged spans
    coalesced, they would always become scavenged. This resulted in most
    spans becoming scavenged over time. While this is good for keeping the
    RSS of the program low, it also causes many more undue page faults and
    many more calls to madvise.
    
    For most applications, the impact of this was negligible. But for
    applications that repeatedly grow and shrink the heap by large amounts,
    the overhead can be significant. The overhead was especially obvious on
    older versions of Linux where MADV_FREE isn't available and
    MADV_DONTNEED must be used.
    
    This change makes it so that scavenged spans will never coalesce with
    unscavenged spans. This  results in fewer page faults overall. Aside
    from this, the expected impact of this change is more heap growths on
    average, as span allocations will be less likely to be fulfilled. To
    mitigate this slightly, this change also coalesces spans eagerly after
    scavenging, to at least ensure that all scavenged spans and all
    unscavenged spans are coalesced with each other.
    
    Also, this change adds additional logic in the case where two adjacent
    spans cannot coalesce. In this case, on platforms where the physical
    page size is larger than the runtime's page size, we realign the
    boundary between the two adjacent spans to a physical page boundary. The
    advantage of this approach is that "unscavengable" spans, that is, spans
    which cannot be scavenged because they don't cover at least a single
    physical page are grown to a size where they have a higher likelihood of
    being discovered by the runtime's scavenging mechanisms when they border
    a scavenged span. This helps prevent the runtime from accruing pockets
    of "unscavengable" memory in between scavenged spans, preventing them
    from coalescing.
    
    We specifically choose to apply this logic to all spans because it
    simplifies the code, even though it isn't strictly necessary. The
    expectation is that this change will result in a slight loss in
    performance on platforms where the physical page size is larger than the
    runtime page size.
    
    Update #14045.
    
    Change-Id: I64fd43eac1d6de6f51d7a2ecb72670f10bb12589
    Reviewed-on: https://go-review.googlesource.com/c/158078
    Run-TryBot: Michael Knyszek <mknyszek@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarAustin Clements <austin@google.com>
    6e9f664b
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