Preserve the 10 threads limit

Signed-off-by: Julien Jerphanion <git@jjerphan.xyz>

Author: jjerphan

libmamba/include/mamba/core/thread_utils.hpp

@@ -73,7 +73,7 @@ namespace mamba
      *
      * The `threads` parameter follows the same semantics as `extract_threads`:
      * - `threads > 0`: user explicitly requested exactly this many threads (never capped);
-     * - `threads == 0`: use the maximum concurrency available to this process;
+     * - `threads == 0`: use `min(10, available_concurrency)` as a conservative default;
      * - `threads < 0`: use (available_concurrency + threads).
      *
      * Only the auto/relative forms (`threads <= 0`) are clamped to the range

libmamba/src/api/configuration.cpp

@@ -1527,7 +1527,8 @@ namespace mamba
                    .set_rc_configurable()
                    .description(
                        "Number of threads for parallel shard fetching (default: 0 (auto)). "
-                       "0 means: use process-affinity-based concurrency."
+                       "If set to 0, the number of threads is chosen automatically as the "
+                       "minimum between 10 and the number of CPUs available to the process"
                    ));
 
         // Network
@@ -1764,9 +1765,8 @@ namespace mamba
                    .description("Defines the number of threads for package download")
                    .long_description(unindent(R"(
                         Defines the number of threads for package download.
-                        If set to 0, the number of threads is chosen automatically
-                        based on the CPUs available to the current process (for example,
-                        when using `taskset`). It has to be non-negative.)")));
+                        If set to 0, the number of threads is chosen automatically as the
+                        minimum between 10 and the number of CPUs available to the process.")));
 
         insert(Configurable("extract_threads", &m_context.threads_params.extract_threads)
                    .group("Extract, Link & Install")
@@ -1778,9 +1778,9 @@ namespace mamba
                         Defines the number of threads for package extraction.
                         Positive values give the exact number of threads.
                         Negative values are interpreted as (available CPUs for this process
-                        minus the absolute value). Zero (the default) means the number of
-                        threads is chosen automatically based on the CPUs available to the
-                        current process (for example, when using `taskset`).)")));
+                        minus the absolute value).
+                        If set to 0, the number of threads is chosen automatically as the
+                        minimum between 10 and the number of CPUs available to the process)")));
 
         insert(Configurable("allow_softlinks", &m_context.link_params.allow_softlinks)
                    .group("Extract, Link & Install")

libmamba/src/core/thread_utils.cpp

@@ -223,7 +223,9 @@ namespace mamba
         std::ptrdiff_t requested = 0;
         if (requested_n_threads == 0)
         {
-            requested = available;
+            // Default to at most 10 threads so that shard-based and other parallel
+            // operations don't oversubscribe very wide machines by default.
+            requested = std::min<std::ptrdiff_t>(available, 10);
         }
         else  // (requested_n_threads < 0)
         {