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5a20a48f13
This shaves off a very significand amount of memory used for evaluation as well as reduces the GC-managed heap. Previously the union discriminator (InternalType) was stored as a separate field in the Value, which takes up whole 8 bytes due to padding needed for member alignment. This effectively wasted 7 whole bytes of memory. Instead of doing that InternalType is instead packed into pointer alignment niches. As it turns out, there's more than enough unused bits there for the bit packing to be effective. See the doxygen comment in the ValueStorage specialization for more details. This does not add any performance overhead, even though we now consistently assert the InternalType in all getters. This can also be made atomic with a double width compare-and-swap instruction on x86_64 (CMPXCHG16B instruction) for parallel evaluation.
127 lines
3.5 KiB
C++
127 lines
3.5 KiB
C++
#include "nix/util/error.hh"
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#include "nix/util/environment-variables.hh"
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#include "nix/expr/eval-settings.hh"
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#include "nix/util/config-global.hh"
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#include "nix/util/serialise.hh"
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#include "nix/expr/eval-gc.hh"
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#include "nix/expr/value.hh"
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#include "expr-config-private.hh"
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#if NIX_USE_BOEHMGC
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# include <pthread.h>
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# ifdef __FreeBSD__
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# include <pthread_np.h>
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# endif
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# include <gc/gc.h>
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# include <gc/gc_cpp.h>
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# include <gc/gc_allocator.h>
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# include <boost/coroutine2/coroutine.hpp>
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# include <boost/coroutine2/protected_fixedsize_stack.hpp>
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# include <boost/context/stack_context.hpp>
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#endif
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namespace nix {
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#if NIX_USE_BOEHMGC
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/* Called when the Boehm GC runs out of memory. */
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static void * oomHandler(size_t requested)
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{
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/* Convert this to a proper C++ exception. */
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throw std::bad_alloc();
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}
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static inline void initGCReal()
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{
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/* Initialise the Boehm garbage collector. */
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/* Don't look for interior pointers. This reduces the odds of
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misdetection a bit. */
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GC_set_all_interior_pointers(0);
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/* We don't have any roots in data segments, so don't scan from
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there. */
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GC_set_no_dls(1);
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/* Enable perf measurements. This is just a setting; not much of a
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start of something. */
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GC_start_performance_measurement();
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GC_INIT();
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/* Register valid displacements in case we are using alignment niches
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for storing the type information. This way tagged pointers are considered
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to be valid, even when they are not aligned. */
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if constexpr (detail::useBitPackedValueStorage<sizeof(void *)>)
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for (std::size_t i = 1; i < sizeof(std::uintptr_t); ++i)
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GC_register_displacement(i);
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GC_set_oom_fn(oomHandler);
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/* Set the initial heap size to something fairly big (25% of
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physical RAM, up to a maximum of 384 MiB) so that in most cases
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we don't need to garbage collect at all. (Collection has a
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fairly significant overhead.) The heap size can be overridden
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through libgc's GC_INITIAL_HEAP_SIZE environment variable. We
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should probably also provide a nix.conf setting for this. Note
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that GC_expand_hp() causes a lot of virtual, but not physical
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(resident) memory to be allocated. This might be a problem on
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systems that don't overcommit. */
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if (!getEnv("GC_INITIAL_HEAP_SIZE")) {
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size_t size = 32 * 1024 * 1024;
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# if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
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size_t maxSize = 384 * 1024 * 1024;
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long pageSize = sysconf(_SC_PAGESIZE);
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long pages = sysconf(_SC_PHYS_PAGES);
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if (pageSize != -1)
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size = (pageSize * pages) / 4; // 25% of RAM
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if (size > maxSize)
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size = maxSize;
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# endif
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debug("setting initial heap size to %1% bytes", size);
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GC_expand_hp(size);
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}
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}
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static size_t gcCyclesAfterInit = 0;
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size_t getGCCycles()
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{
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assertGCInitialized();
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return static_cast<size_t>(GC_get_gc_no()) - gcCyclesAfterInit;
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}
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#endif
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static bool gcInitialised = false;
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void initGC()
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{
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if (gcInitialised)
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return;
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#if NIX_USE_BOEHMGC
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initGCReal();
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gcCyclesAfterInit = GC_get_gc_no();
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#endif
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// NIX_PATH must override the regular setting
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// See the comment in applyConfig
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if (auto nixPathEnv = getEnv("NIX_PATH")) {
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globalConfig.set("nix-path", concatStringsSep(" ", EvalSettings::parseNixPath(nixPathEnv.value())));
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}
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gcInitialised = true;
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}
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void assertGCInitialized()
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{
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assert(gcInitialised);
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}
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} // namespace nix
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