mongo::SecureRandom() holds a std::ifstream to read from /dev/urandom.
std::ifstream is buffered by default, so the first use of the SecureRandom fills that 8kiB buffer. This is extremely wasteful, as a SecureRandom object is often used for a few words and discarded, and /dev/urandom entropy and/or computational load is a resource we don't want to squander unnecessarily.

As a patch, we can reduce the buffering to maybe 64 bytes or so, possibly as part of SERVER-43641 which is in there anyway. Turning off buffering completely would incur read() syscalls on every use, which could be risky, so a 100X smaller buffer seems a good tradeoff.

https://gist.github.com/BillyDonahue/53fa229d311cc6ac1f5ab21bd588e11f#file-gistfile2-txt

Catchpoint 5 (call to syscall read), 0x00007ffff4e0534e in __libc_read (fd=6, buf=buf@entry=0x7fffeee07820, nbytes=nbytes@entry=8191) at ../sysdeps/unix/sysv/linux/read.c:27
27 ../sysdeps/unix/sysv/linux/read.c: No such file or directory.
(gdb) bt
#0 0x00007ffff4e0534e in __libc_read (fd=6, buf=buf@entry=0x7fffeee07820, nbytes=nbytes@entry=8191) at ../sysdeps/unix/sysv/linux/read.c:27
#1 0x00007ffff7916287 in std::_basic_file<char>::xsgetn (this=this@entry=0x7fffeee004d8, __s=0x7fffeee07820 '\253' <repeats 200 times>..., __n=_n@entry=8191) at basic_file.cc:285
#2 0x00007ffff793ff30 in std::basic_filebuf<char, std::char_traits<char> >::underflow (this=0x7fffeee00470) at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/bits/char_traits.h:350
#3 0x00007ffff78ee576 in std::basic_streambuf<char, std::char_traits<char> >::uflow (this=0x7fffeee00470) at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/streambuf:707
#4 std::basic_streambuf<char, std::char_traits<char> >::xsgetn (this=this@entry=0x7fffeee00470, _s=s@entry=0x7fffffffd468 " \362\333\356\377\177", __n=_n@entry=8)
at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/bits/streambuf.tcc:64
#5 0x00007ffff79409e3 in std::basic_filebuf<char, std::char_traits<char> >::xsgetn (this=0x7fffeee00470, __s=0x7fffffffd468 " \362\333\356\377\177", __n=8)
at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/bits/codecvt.h:210
#6 0x00007ffff78e989d in std::basic_streambuf<char, std::char_traits<char> >::sgetn (__n=8, __s=0x7fffffffd468 " \362\333\356\377\177", this=<optimized out>)
at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/streambuf:364
#7 std::istream::read (this=0x7fffeee00460, __s=0x7fffffffd468 " \362\333\356\377\177", __n=8) at /home/billy/dev/10gen/toolchain-builder/tmp/build-gcc-v3.sh-sRY/build/x86_64-mongodb-linux/libstdc++-v3/include/bits/istream.tcc:667
#8 0x00007ffff7c9b2be in mongo::random_detail::SecureUrbg::State::operator() (this=0x7fffeee00460) at src/mongo/platform/random.cpp:127
#9 0x00007ffff7c9b02c in mongo::random_detail::SecureUrbg::operator() (this=0x7fffeedbf220) at src/mongo/platform/random.cpp:139
#10 0x00005555557397ee in std::uniform_int_distribution<long>::operator()<mongo::random_detail::SecureUrbg> (this=0x7fffffffd5f0, __urng=..., __param=...) at /opt/mongodbtoolchain/stow/gcc-v3.sFn/include/c++/8.2.0/bits/uniform_int_dist.h:275
#11 0x0000555555736bd3 in std::uniform_int_distribution<long>::operator()<mongo::random_detail::SecureUrbg> (this=0x7fffffffd5f0, __urng=...) at /opt/mongodbtoolchain/stow/gcc-v3.sFn/include/c++/8.2.0/bits/uniform_int_dist.h:166
#12 0x0000555555733f35 in mongo::RandomBase<mongo::random_detail::SecureUrbg>::_nextAny<long> (this=0x7fffeedbf220) at src/mongo/platform/random.h:137
#13 0x000055555572f578 in mongo::RandomBase<mongo::random_detail::SecureUrbg>::nextInt64 (this=0x7fffeedbf220) at src/mongo/platform/random.h:120
#14 0x00007ffff7c7d232 in mongo::_mongoInitializerFunction_OIDGeneration (context=0x7fffffffd7b0) at src/mongo/bson/oid.cpp:58
...
#21 0x00007ffff7fe51f3 in main (argc=3, argv=0x7fffffffddb8, envp=0x7fffffffddd8) at src/mongo/unittest/unittest_main.cpp:52
#22 0x00007ffff4a24b97 in __libc_start_main (main=0x7ffff7fe519c <main(int, char*, char*)>, argc=3, argv=0x7fffffffddb8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdda8) at ../csu/libc-start.c:310
#23 0x00005555556f9cea in _start ()
(gdb)