smp: disable mlock on overcommit

Memory locking keeps pages in physical memory, and decreases the amount of pages available for swapping on an overcommit setup. Thus, it can decrease fairness, increase memory pressure, and cause OOMs. So, it is less suitable for a cooperative overcommit setup.

This change disables memory locking on overcommit (overprovisioned - I hope I can rename that config). Also, it reuses the mlock variable. I assume that this change shouldn't break reasonable configs.

include/seastar/core/reactor_config.hh

@@ -131,6 +131,7 @@ struct reactor_options : public program_options::option_group {
     /// * \ref idle_poll_time_us = 0
     /// * \ref smp_options::thread_affinity = 0
     /// * \ref poll_aio = 0
+    /// * \ref smp_options::lock_memory = 0
     program_options::value<> overprovisioned;
     /// \brief Abort when seastar allocator cannot allocate memory.
     program_options::value<> abort_on_seastar_bad_alloc;

include/seastar/core/smp_options.hh

@@ -57,6 +57,8 @@ struct smp_options : public program_options::option_group {
     /// Path to accessible hugetlbfs mount (typically /dev/hugepages/something).
     program_options::value<std::string> hugepages;
     /// Lock all memory (prevents swapping).
+    ///
+    /// Disable for overprovisioned environments.
     program_options::value<bool> lock_memory;
     /// Pin threads to their cpus (disable for overprovisioning).
     ///

src/core/reactor.cc

@@ -3814,7 +3814,7 @@ smp_options::smp_options(program_options::option_group* parent_group)
     , memory(*this, "memory", std::nullopt, "memory to use, in bytes (ex: 4G) (default: all)")
     , reserve_memory(*this, "reserve-memory", {}, "memory reserved to OS (if --memory not specified)")
     , hugepages(*this, "hugepages", {}, "path to accessible hugetlbfs mount (typically /dev/hugepages/something)")
-    , lock_memory(*this, "lock-memory", {}, "lock all memory (prevents swapping)")
+    , lock_memory(*this, "lock-memory", {}, "lock all memory (prevents swapping). Disable for overprovisioned environments")
     , thread_affinity(*this, "thread-affinity", true, "pin threads to their cpus (disable for overprovisioning)")
 #ifdef SEASTAR_HAVE_HWLOC
     , num_io_groups(*this, "num-io-groups", {}, "Number of IO groups. Each IO group will be responsible for a fraction of the IO requests. Defaults to the number of NUMA nodes")
@@ -4332,7 +4332,7 @@ void smp::configure(const smp_options& smp_opts, const reactor_options& reactor_
         hugepages_path = smp_opts.hugepages.get_value();
     }
     auto mlock = false;
-    if (smp_opts.lock_memory) {
+    if (smp_opts.lock_memory && !reactor_opts.overprovisioned) {
         mlock = smp_opts.lock_memory.get_value();
     }
     if (mlock) {
@@ -4603,7 +4603,7 @@ void smp::configure(const smp_options& smp_opts, const reactor_options& reactor_
 
     engine().configure(reactor_opts);
 
-    if (smp_opts.lock_memory && smp_opts.lock_memory.get_value() && layout && !layout->ranges.empty()) {
+    if (mlock && layout && !layout->ranges.empty()) {
         smp::setup_prefaulter(resources, std::move(*layout));
     }
 }