* [RFC] TUNING: document THP caveat for Linux users
@ 2016-11-29 0:00 Eric Wong
0 siblings, 0 replies; only message in thread
From: Eric Wong @ 2016-11-29 0:00 UTC (permalink / raw)
This probably applies to other kernels, too, but I'm most
familiar with Linux.
It took me a while to get the wording below to this point.
Maybe there's not enough detail for folks unfamiliar with
how OSes work, or maybe there's too much and will be TL;DR-ed...
TUNING | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/TUNING b/TUNING
index 247090b..1f55228 100644
@@ -72,10 +72,28 @@ See Unicorn::Configurator for details on the config file format.
have them unbuffered (File#sync = true) or they are
record(line)-buffered in userspace before any writes.
-== Kernel Parameters (Linux sysctl)
+== Kernel Parameters (Linux sysctl and sysfs)
WARNING: Do not change system parameters unless you know what you're doing!
+* Transparent hugepages (THP) improves performance in many cases,
+ but can also increase memory use when relying on a
+ copy-on-write(CoW)-friendly GC (Ruby 2.0+) with "preload_app true".
+ CoW operates at the page level, so writing to a huge page would
+ trigger a 2 MB copy (x86-64), as opposed to a 4 KB copy on a
+ regular (non-huge) page.
+ Consider only allowing THP to be used when requested via the
+ madvise(2) syscall:
+ echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
+ Or disabling it system-wide, via "never".
+ n.b. "page" in this context only applies to the OS kernel,
+ Ruby GC implementations also use this term for the same concept
+ in a way that is agnostic to the OS.
* net.core.rmem_max and net.core.wmem_max can increase the allowed
size of :rcvbuf and :sndbuf respectively. This is mostly only useful
for UNIX domain sockets which do not have auto-tuning buffer sizes.
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2016-11-29 0:00 [RFC] TUNING: document THP caveat for Linux users Eric Wong
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