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Avoid buffered reads of /dev/urandom

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I am seeing disappointing clang performance on a large PowerPC64
Linux box. GetRandomNumberSeed() does a buffered read from
/dev/urandom to seed its PRNG. As a result we read an entire page
even though we only need 4 bytes.

With every clang task reading a page worth of /dev/urandom we
end up spending a large amount of time stuck on kernel spinlock.

Patch by Anton Blanchard!

llvm-svn: 255386
This commit is contained in:
Rafael Espindola 2015-12-11 22:52:32 +00:00
parent 5f81084374
commit 8ea226400f
1 changed files with 9 additions and 4 deletions
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13
lib/Support/Unix/Process.inc
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@ -430,13 +430,18 @@ const char *Process::ResetColor() {
#if !defined(HAVE_DECL_ARC4RANDOM) || !HAVE_DECL_ARC4RANDOM
static unsigned GetRandomNumberSeed() {
// Attempt to get the initial seed from /dev/urandom, if possible.
if (FILE *RandomSource = ::fopen("/dev/urandom", "r")) {
int urandomFD = open("/dev/urandom", O_RDONLY);
if (urandomFD != -1) {
unsigned seed;
int count = ::fread((void *)&seed, sizeof(seed), 1, RandomSource);
::fclose(RandomSource);
// Don't use a buffered read to avoid reading more data
// from /dev/urandom than we need.
int count = read(urandomFD, (void *)&seed, sizeof(seed));
close(urandomFD);
// Return the seed if the read was successful.
if (count == 1)
if (count == sizeof(seed))
return seed;
}