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Factor out GC initialization code

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This is not really part of the evaluator: it is just an integration
between Boehm GC and Boost coroutines usable for any purpose. The
evaluator (merely) optionally uses it.
This commit is contained in:
John Ericson 2024-06-12 10:17:39 -04:00
parent 7c2981fc55
commit 5b53d8fec3
4 changed files with 249 additions and 210 deletions
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226
src/libexpr/eval-gc.cc Normal file
View file

@ -0,0 +1,226 @@
#include "error.hh"
#include "environment-variables.hh"
#include "serialise.hh"
#include "eval-gc.hh"
#if HAVE_BOEHMGC
# include <pthread.h>
# if __FreeBSD__
# include <pthread_np.h>
# endif
# include <gc/gc.h>
# include <gc/gc_cpp.h>
# include <gc/gc_allocator.h>
# include <boost/coroutine2/coroutine.hpp>
# include <boost/coroutine2/protected_fixedsize_stack.hpp>
# include <boost/context/stack_context.hpp>
#endif
namespace nix {
#if HAVE_BOEHMGC
/* Called when the Boehm GC runs out of memory. */
static void * oomHandler(size_t requested)
{
/* Convert this to a proper C++ exception. */
throw std::bad_alloc();
}
class BoehmGCStackAllocator : public StackAllocator
{
boost::coroutines2::protected_fixedsize_stack stack{
// We allocate 8 MB, the default max stack size on NixOS.
// A smaller stack might be quicker to allocate but reduces the stack
// depth available for source filter expressions etc.
std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))};
// This is specific to boost::coroutines2::protected_fixedsize_stack.
// The stack protection page is included in sctx.size, so we have to
// subtract one page size from the stack size.
std::size_t pfss_usable_stack_size(boost::context::stack_context & sctx)
{
return sctx.size - boost::context::stack_traits::page_size();
}
public:
boost::context::stack_context allocate() override
{
auto sctx = stack.allocate();
// Stacks generally start at a high address and grow to lower addresses.
// Architectures that do the opposite are rare; in fact so rare that
// boost_routine does not implement it.
// So we subtract the stack size.
GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
return sctx;
}
void deallocate(boost::context::stack_context sctx) override
{
GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
stack.deallocate(sctx);
}
};
static BoehmGCStackAllocator boehmGCStackAllocator;
/**
* When a thread goes into a coroutine, we lose its original sp until
* control flow returns to the thread.
* While in the coroutine, the sp points outside the thread stack,
* so we can detect this and push the entire thread stack instead,
* as an approximation.
* The coroutine's stack is covered by `BoehmGCStackAllocator`.
* This is not an optimal solution, because the garbage is scanned when a
* coroutine is active, for both the coroutine and the original thread stack.
* However, the implementation is quite lean, and usually we don't have active
* coroutines during evaluation, so this is acceptable.
*/
void fixupBoehmStackPointer(void ** sp_ptr, void * _pthread_id)
{
void *& sp = *sp_ptr;
auto pthread_id = reinterpret_cast<pthread_t>(_pthread_id);
pthread_attr_t pattr;
size_t osStackSize;
void * osStackLow;
void * osStackBase;
# ifdef __APPLE__
osStackSize = pthread_get_stacksize_np(pthread_id);
osStackLow = pthread_get_stackaddr_np(pthread_id);
# else
if (pthread_attr_init(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
}
# ifdef HAVE_PTHREAD_GETATTR_NP
if (pthread_getattr_np(pthread_id, &pattr)) {
throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
}
# elif HAVE_PTHREAD_ATTR_GET_NP
if (!pthread_attr_init(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
}
if (!pthread_attr_get_np(pthread_id, &pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_get_np failed");
}
# else
# error "Need one of `pthread_attr_get_np` or `pthread_getattr_np`"
# endif
if (pthread_attr_getstack(&pattr, &osStackLow, &osStackSize)) {
throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
}
if (pthread_attr_destroy(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
}
# endif
osStackBase = (char *) osStackLow + osStackSize;
// NOTE: We assume the stack grows down, as it does on all architectures we support.
// Architectures that grow the stack up are rare.
if (sp >= osStackBase || sp < osStackLow) { // lo is outside the os stack
sp = osStackBase;
}
}
/* Disable GC while this object lives. Used by CoroutineContext.
*
* Boehm keeps a count of GC_disable() and GC_enable() calls,
* and only enables GC when the count matches.
*/
class BoehmDisableGC
{
public:
BoehmDisableGC()
{
GC_disable();
};
~BoehmDisableGC()
{
GC_enable();
};
};
static inline void initGCReal()
{
/* Initialise the Boehm garbage collector. */
/* Don't look for interior pointers. This reduces the odds of
misdetection a bit. */
GC_set_all_interior_pointers(0);
/* We don't have any roots in data segments, so don't scan from
there. */
GC_set_no_dls(1);
GC_INIT();
GC_set_oom_fn(oomHandler);
StackAllocator::defaultAllocator = &boehmGCStackAllocator;
// TODO: Remove __APPLE__ condition.
// Comment suggests an implementation that works on darwin and windows
// https://github.com/ivmai/bdwgc/issues/362#issuecomment-1936672196
# if GC_VERSION_MAJOR >= 8 && GC_VERSION_MINOR >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
GC_set_sp_corrector(&fixupBoehmStackPointer);
if (!GC_get_sp_corrector()) {
printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
/* Used to disable GC when entering coroutines on macOS */
create_coro_gc_hook = []() -> std::shared_ptr<void> { return std::make_shared<BoehmDisableGC>(); };
}
# else
# warning \
"BoehmGC version does not support GC while coroutine exists. GC will be disabled inside coroutines. Consider updating bdw-gc to 8.2.4 or later."
# endif
/* Set the initial heap size to something fairly big (25% of
physical RAM, up to a maximum of 384 MiB) so that in most cases
we don't need to garbage collect at all. (Collection has a
fairly significant overhead.) The heap size can be overridden
through libgc's GC_INITIAL_HEAP_SIZE environment variable. We
should probably also provide a nix.conf setting for this. Note
that GC_expand_hp() causes a lot of virtual, but not physical
(resident) memory to be allocated. This might be a problem on
systems that don't overcommit. */
if (!getEnv("GC_INITIAL_HEAP_SIZE")) {
size_t size = 32 * 1024 * 1024;
# if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
size_t maxSize = 384 * 1024 * 1024;
long pageSize = sysconf(_SC_PAGESIZE);
long pages = sysconf(_SC_PHYS_PAGES);
if (pageSize != -1)
size = (pageSize * pages) / 4; // 25% of RAM
if (size > maxSize)
size = maxSize;
# endif
debug("setting initial heap size to %1% bytes", size);
GC_expand_hp(size);
}
}
#endif
static bool gcInitialised = false;
void initGC()
{
if (gcInitialised)
return;
#if HAVE_BOEHMGC
initGCReal();
#endif
gcInitialised = true;
}
void assertGCInitialized()
{
assert(gcInitialised);
}
}

16
src/libexpr/eval-gc.hh Normal file
View file

@ -0,0 +1,16 @@
#pragma once
///@file
namespace nix {
/**
* Initialise the Boehm GC, if applicable.
*/
void initGC();
/**
* Make sure `initGC` has already been called.
*/
void assertGCInitialized();
}

210
src/libexpr/eval.cc
View file

@ -40,25 +40,16 @@
#include <boost/container/small_vector.hpp>
#ifndef _WIN32 // TODO use portable implementation
# include <sys/resource.h>
# include <sys/resource.h>
#endif
#if HAVE_BOEHMGC
#define GC_INCLUDE_NEW
# define GC_INCLUDE_NEW
#include <pthread.h>
#if __FreeBSD__
# include <pthread_np.h>
#endif
#include <gc/gc.h>
#include <gc/gc_cpp.h>
#include <gc/gc_allocator.h>
#include <boost/coroutine2/coroutine.hpp>
#include <boost/coroutine2/protected_fixedsize_stack.hpp>
#include <boost/context/stack_context.hpp>
# include <gc/gc.h>
# include <gc/gc_cpp.h>
# include <gc/gc_allocator.h>
#endif
@ -211,109 +202,6 @@ bool Value::isTrivial() const
}
#if HAVE_BOEHMGC
/* Called when the Boehm GC runs out of memory. */
static void * oomHandler(size_t requested)
{
/* Convert this to a proper C++ exception. */
throw std::bad_alloc();
}
class BoehmGCStackAllocator : public StackAllocator {
boost::coroutines2::protected_fixedsize_stack stack {
// We allocate 8 MB, the default max stack size on NixOS.
// A smaller stack might be quicker to allocate but reduces the stack
// depth available for source filter expressions etc.
std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))
};
// This is specific to boost::coroutines2::protected_fixedsize_stack.
// The stack protection page is included in sctx.size, so we have to
// subtract one page size from the stack size.
std::size_t pfss_usable_stack_size(boost::context::stack_context &sctx) {
return sctx.size - boost::context::stack_traits::page_size();
}
public:
boost::context::stack_context allocate() override {
auto sctx = stack.allocate();
// Stacks generally start at a high address and grow to lower addresses.
// Architectures that do the opposite are rare; in fact so rare that
// boost_routine does not implement it.
// So we subtract the stack size.
GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
return sctx;
}
void deallocate(boost::context::stack_context sctx) override {
GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
stack.deallocate(sctx);
}
};
static BoehmGCStackAllocator boehmGCStackAllocator;
/**
* When a thread goes into a coroutine, we lose its original sp until
* control flow returns to the thread.
* While in the coroutine, the sp points outside the thread stack,
* so we can detect this and push the entire thread stack instead,
* as an approximation.
* The coroutine's stack is covered by `BoehmGCStackAllocator`.
* This is not an optimal solution, because the garbage is scanned when a
* coroutine is active, for both the coroutine and the original thread stack.
* However, the implementation is quite lean, and usually we don't have active
* coroutines during evaluation, so this is acceptable.
*/
void fixupBoehmStackPointer(void ** sp_ptr, void * _pthread_id) {
void *& sp = *sp_ptr;
auto pthread_id = reinterpret_cast<pthread_t>(_pthread_id);
pthread_attr_t pattr;
size_t osStackSize;
void * osStackLow;
void * osStackBase;
# ifdef __APPLE__
osStackSize = pthread_get_stacksize_np(pthread_id);
osStackLow = pthread_get_stackaddr_np(pthread_id);
# else
if (pthread_attr_init(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
}
# ifdef HAVE_PTHREAD_GETATTR_NP
if (pthread_getattr_np(pthread_id, &pattr)) {
throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
}
# elif HAVE_PTHREAD_ATTR_GET_NP
if (!pthread_attr_init(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
}
if (!pthread_attr_get_np(pthread_id, &pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_get_np failed");
}
# else
# error "Need one of `pthread_attr_get_np` or `pthread_getattr_np`"
# endif
if (pthread_attr_getstack(&pattr, &osStackLow, &osStackSize)) {
throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
}
if (pthread_attr_destroy(&pattr)) {
throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
}
# endif
osStackBase = (char *)osStackLow + osStackSize;
// NOTE: We assume the stack grows down, as it does on all architectures we support.
// Architectures that grow the stack up are rare.
if (sp >= osStackBase || sp < osStackLow) { // lo is outside the os stack
sp = osStackBase;
}
}
#endif
static Symbol getName(const AttrName & name, EvalState & state, Env & env)
{
if (name.symbol) {
@ -326,92 +214,6 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
}
}
#if HAVE_BOEHMGC
/* Disable GC while this object lives. Used by CoroutineContext.
*
* Boehm keeps a count of GC_disable() and GC_enable() calls,
* and only enables GC when the count matches.
*/
class BoehmDisableGC {
public:
BoehmDisableGC() {
GC_disable();
};
~BoehmDisableGC() {
GC_enable();
};
};
#endif
static bool gcInitialised = false;
void initGC()
{
if (gcInitialised) return;
#if HAVE_BOEHMGC
/* Initialise the Boehm garbage collector. */
/* Don't look for interior pointers. This reduces the odds of
misdetection a bit. */
GC_set_all_interior_pointers(0);
/* We don't have any roots in data segments, so don't scan from
there. */
GC_set_no_dls(1);
GC_INIT();
GC_set_oom_fn(oomHandler);
StackAllocator::defaultAllocator = &boehmGCStackAllocator;
// TODO: Remove __APPLE__ condition.
// Comment suggests an implementation that works on darwin and windows
// https://github.com/ivmai/bdwgc/issues/362#issuecomment-1936672196
#if GC_VERSION_MAJOR >= 8 && GC_VERSION_MINOR >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
GC_set_sp_corrector(&fixupBoehmStackPointer);
if (!GC_get_sp_corrector()) {
printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
/* Used to disable GC when entering coroutines on macOS */
create_coro_gc_hook = []() -> std::shared_ptr<void> {
return std::make_shared<BoehmDisableGC>();
};
}
#else
#warning "BoehmGC version does not support GC while coroutine exists. GC will be disabled inside coroutines. Consider updating bdw-gc to 8.2.4 or later."
#endif
/* Set the initial heap size to something fairly big (25% of
physical RAM, up to a maximum of 384 MiB) so that in most cases
we don't need to garbage collect at all. (Collection has a
fairly significant overhead.) The heap size can be overridden
through libgc's GC_INITIAL_HEAP_SIZE environment variable. We
should probably also provide a nix.conf setting for this. Note
that GC_expand_hp() causes a lot of virtual, but not physical
(resident) memory to be allocated. This might be a problem on
systems that don't overcommit. */
if (!getEnv("GC_INITIAL_HEAP_SIZE")) {
size_t size = 32 * 1024 * 1024;
#if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
size_t maxSize = 384 * 1024 * 1024;
long pageSize = sysconf(_SC_PAGESIZE);
long pages = sysconf(_SC_PHYS_PAGES);
if (pageSize != -1)
size = (pageSize * pages) / 4; // 25% of RAM
if (size > maxSize) size = maxSize;
#endif
debug("setting initial heap size to %1% bytes", size);
GC_expand_hp(size);
}
#endif
gcInitialised = true;
}
static constexpr size_t BASE_ENV_SIZE = 128;
EvalState::EvalState(
@ -508,7 +310,7 @@ EvalState::EvalState(
countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
assert(gcInitialised);
assertGCInitialized();
static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");

7
src/libexpr/eval.hh
View file

@ -3,6 +3,7 @@
#include "attr-set.hh"
#include "eval-error.hh"
#include "eval-gc.hh"
#include "types.hh"
#include "value.hh"
#include "nixexpr.hh"
@ -146,12 +147,6 @@ std::string printValue(EvalState & state, Value & v);
std::ostream & operator << (std::ostream & os, const ValueType t);
/**
* Initialise the Boehm GC, if applicable.
*/
void initGC();
struct RegexCache;
std::shared_ptr<RegexCache> makeRegexCache();