62 lines
2.3 KiB
Diff
62 lines
2.3 KiB
Diff
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ghc runtime by default (otherwise depending on a "configure" option)
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does memory allocation on their own by first mmapping a 1 TB range of
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memory into the process and then parceling out chunks from it.
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If one of the chunks is not needed, the kernel needs to be informed -
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otherwise the system would quickly run out of available RAM.
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ghc does that via madvise(2).
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There are two options when doing this informing:
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MADV_FREE - Means "I don't need this range or the data in it any more".
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Kernel promises to fail later accesses to it.
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MADV_DONTNEED - Means "I don't need this range right now - and I don't
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need the data in it anymore". Kernel promises to make later accesses to
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it succeed (if necessary by providing a new page initialized with zeroes).
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MADV_FREE was introduced in Linux 4.5.
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glibc 2.25 and later always define MADV_FREE.
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Unpatched ghc 8.0.2 will use either MADV_FREE or MADV_DONTNEED, determined
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at ghc compile time. Which of them will actually succeed is determined
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by the Linux kernel at run time.
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This patch makes ghc try MADV_FREE. If it doesn't work, it falls back to
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MADV_DONTNEED.
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The end result is that ghc programs free their memory with Linux < 4.5 again.
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See https://git.haskell.org/ghc.git/commitdiff/6576bf83cdf4eac05eb88a24aa934a736c91e3da for more information.
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--- a/rts/posix/OSMem.c
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+++ b/rts/posix/OSMem.c
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@@ -541,11 +541,24 @@ void osDecommitMemory(void *at, W_ size)
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#ifdef MADV_FREE
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// Try MADV_FREE first, FreeBSD has both and MADV_DONTNEED
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- // just swaps memory out
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+ // just swaps memory out. Linux >= 4.5 has both DONTNEED and FREE; either
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+ // will work as they both allow the system to free anonymous pages.
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+ // It is important that we try both methods as the kernel which we were
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+ // built on may differ from the kernel we are now running on.
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r = madvise(at, size, MADV_FREE);
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-#else
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- r = madvise(at, size, MADV_DONTNEED);
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+ if(r < 0) {
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+ if (errno == EINVAL) {
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+ // Perhaps the system doesn't support MADV_FREE; fall-through and
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+ // try MADV_DONTNEED.
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+ } else {
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+ sysErrorBelch("unable to decommit memory");
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+ }
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+ } else {
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+ return;
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+ }
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#endif
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+
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+ r = madvise(at, size, MADV_DONTNEED);
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if(r < 0)
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sysErrorBelch("unable to decommit memory");
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}
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