Derive hashmap head buckets from RAM and expected count.

include/bitcoin/database/memory/utilities.hpp

@@ -33,6 +33,12 @@ BCD_API size_t page_size() NOEXCEPT;
 /// The bytes of physical memory, zero if failed.
 BCD_API uint64_t system_memory() NOEXCEPT;
 
+/// Head buckets from expected elements and installed memory, scaled linearly
+/// between the contested and uncontested load factors (in tenths). Hashmaps
+/// only; zero expected is not derived.
+BCD_API uint32_t derive_buckets(uint64_t expected, uint32_t low,
+    uint32_t high) NOEXCEPT;
+
 /// The bytes of unused physical memory, zero if failed. Scarcity precedes
 /// pressure: clean file cache is reclaimable, so the pressure level does not
 /// raise while free memory exhausts.
@@ -51,10 +57,12 @@ BCD_API size_t system_pressure() NOEXCEPT;
 /// if failed or the platform provides no source.
 BCD_API uint64_t system_compressed() NOEXCEPT;
 
+// TODO: move all below to release.hpp, remove compound ternaries, delint.
+
 /// Head-release page-run helpers (pure, unit tested). Pages are tracked as
 /// bits in 64-bit words (low order bit is the lowest page of the word). A
 /// release candidate page is clean (not dirty), cold (not recently written)
-/// and not already released; candidacy is limited to full pages below the
+/// and not already released. Candidacy is limited to full pages below the
 /// logical page count.
 /// ---------------------------------------------------------------------------
 
@@ -134,20 +142,6 @@ inline std::pair<size_t, size_t> bit_run(const uint64_t* words, size_t pages,
     return { start, end };
 }
 
-/// C++26: std::atomic<size_t>::fetch_max
-template <typename Integral, if_integral_integer<Integral> = true>
-Integral fetch_max(std::atomic<Integral>& atomic, Integral value) NOEXCEPT
-{
-    constexpr auto relaxed = std::memory_order_relaxed;
-
-    auto atom = atomic.load(relaxed);
-    while (value > atom && !atomic.compare_exchange_weak(atom, value, relaxed))
-    {
-    }
-
-    return atom;
-}
-
 } // namespace database
 } // namespace libbitcoin
 

src/memory/utilities.cpp

@@ -37,15 +37,16 @@
 namespace libbitcoin {
 namespace database {
 
+using namespace system;
+
 #if defined(HAVE_MSC)
 
 size_t page_size() NOEXCEPT
 {
-    using namespace system;
     SYSTEM_INFO info{};
     ::GetSystemInfo(&info);
 
-    BC_ASSERT(!system::is_limited<size_t>(info.dwPageSize));
+    BC_ASSERT(!is_limited<size_t>(info.dwPageSize));
     return info.dwPageSize;
 }
 
@@ -94,12 +95,11 @@ uint64_t system_compressed() NOEXCEPT
     return zero;
 }
 
-#else
+#else // HAVE_MSC
 
 size_t page_size() NOEXCEPT
 {
     errno = 0;
-    using namespace system;
     const auto size = sysconf(_SC_PAGESIZE);
     if (is_zero(errno) && !is_negative(size))
     {
@@ -113,7 +113,6 @@ size_t page_size() NOEXCEPT
 uint64_t system_memory() NOEXCEPT
 {
     errno = 0;
-    using namespace system;
     const int64_t pages = sysconf(_SC_PHYS_PAGES);
     if (is_zero(errno) && !is_negative(pages))
     {
@@ -128,7 +127,6 @@ uint64_t system_memory() NOEXCEPT
 uint64_t system_free() NOEXCEPT
 {
 #if defined(HAVE_APPLE)
-    using namespace system;
     auto count = HOST_VM_INFO64_COUNT;
     vm_statistics64_data_t statistics{};
     if (::host_statistics64(::mach_host_self(), HOST_VM_INFO64,
@@ -138,7 +136,6 @@ uint64_t system_free() NOEXCEPT
     }
 #else
     errno = 0;
-    using namespace system;
     const int64_t pages = sysconf(_SC_AVPHYS_PAGES);
     if (is_zero(errno) && !is_negative(pages))
     {
@@ -157,7 +154,6 @@ uint64_t system_available() NOEXCEPT
 #if defined(HAVE_APPLE)
     // Free plus purgeable plus file-backed (external) pages are available to
     // allocation without compression or swap.
-    using namespace system;
     auto count = HOST_VM_INFO64_COUNT;
     vm_statistics64_data_t statistics{};
     if (::host_statistics64(::mach_host_self(), HOST_VM_INFO64,
@@ -180,7 +176,7 @@ uint64_t system_available() NOEXCEPT
                 &kilobytes) == 1)
             {
                 std::fclose(file);
-                return system::ceilinged_multiply<uint64_t>(kilobytes, 1024u);
+                return ceilinged_multiply<uint64_t>(kilobytes, 1024u);
             }
         }
 
@@ -200,7 +196,7 @@ size_t system_pressure() NOEXCEPT
     if (is_zero(::sysctlbyname("kern.memorystatus_vm_pressure_level", &level,
         &size, nullptr, 0)))
     {
-        return system::possible_narrow_sign_cast<size_t>(level);
+        return possible_narrow_sign_cast<size_t>(level);
     }
 #elif defined(HAVE_LINUX)
     // PSI (requires CONFIG_PSI): fraction of recent wall time that tasks
@@ -235,7 +231,6 @@ size_t system_pressure() NOEXCEPT
 uint64_t system_compressed() NOEXCEPT
 {
 #if defined(HAVE_APPLE)
-    using namespace system;
     auto count = HOST_VM_INFO64_COUNT;
     vm_statistics64_data_t statistics{};
     if (::host_statistics64(::mach_host_self(), HOST_VM_INFO64,
@@ -251,7 +246,41 @@ uint64_t system_compressed() NOEXCEPT
     return zero;
 }
 
+#endif // HAVE_MSC
+
+constexpr auto gigabyte = power2<uint64_t>(30u);
+constexpr auto megabyte = power2<uint64_t>(20u);
+constexpr auto uncontested = power2<uint64_t>(35u);
+
+#if defined(HAVE_APPLE)
+constexpr auto contested = 10u * gigabyte;
+#else
+constexpr auto contested = 8u * gigabyte;
 #endif
 
+uint32_t derive_buckets(uint64_t expected, uint32_t low,
+    uint32_t high) NOEXCEPT
+{
+    if (is_zero(expected) || is_zero(low) || is_zero(high))
+        return {};
+
+    const auto scaled = ceilinged_multiply<uint64_t>(expected, 10);
+    const auto floored = scaled / low;
+    const auto ceiled = scaled / high;
+    const auto memory = system_memory();
+
+    if (memory <= contested)
+        return possible_narrow_cast<uint32_t>(floored);
+
+    if (memory >= uncontested)
+        return possible_narrow_cast<uint32_t>(ceiled);
+
+    // Megabytes, as the byte product overflows the word.
+    const auto over = (memory - contested) / megabyte;
+    const auto rise = floored_subtract(ceiled, floored);
+    constexpr auto span = (uncontested - contested) / megabyte;
+    return limit<uint32_t>(floored + (ceilinged_multiply(rise, over) / span));
+}
+
 } // namespace database
 } // namespace libbitcoin