system: make GetTotalRAM() cgroup-aware on Linux #34762

1. 
   ViniciusCestarii commented at 6:44 pm on March 6, 2026: none

   sysconf(_SC_PHYS_PAGES) * sysconf(_SC_PAGE_SIZE) reports host-wide physical memory, ignoring cgroup limits. Inside containers, this causes GetTotalRAM() to return the full host RAM instead of what the process is actually allowed to use.

   This PR caps the reported value with the cgroup memory limit by reading cgroup v2 (memory.max) or v1 (memory.limit_in_bytes) on Linux. When no limit is set, the host-wide value is kept unchanged.

   It also helps with #34692 where dbcache defaults depend on detected system RAM.

2. 
   DrahtBot commented at 6:45 pm on March 6, 2026: contributor

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3. in src/common/system.cpp:33 in 960c817e5e

   29@@ -29,6 +30,7 @@
   30 #include <cstddef>
   31 #include <cstdint>
   32 #include <cstdlib>
   33+#include <fstream>
   

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   brunoerg commented at 6:57 pm on March 6, 2026:

   960c817e5eb3bffad4b0ff42e6656dfa4b1a8659: It is only needed in __linux__ path, right? So it is not needed on all platforms.

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   ViniciusCestarii commented at 7:13 pm on March 6, 2026:

   Yes, #include <util/strencodings.h> is too. I fixed both
4. system: make GetTotalRAM() cgroup-aware on Linux e12342c585
5. in src/common/system.cpp:143 in 960c817e5e

   139+            if (cg2 >> val && val != "max") {
   140+                if (auto limit{ToIntegral<uint64_t>(val)}) physical_ram = std::min(physical_ram, *limit);
   141+            }
   142+        } else if (std::ifstream cg1{"/sys/fs/cgroup/memory/memory.limit_in_bytes"}; cg1) {
   143+            uint64_t cgroup_limit;
   144+            if (cg1 >> cgroup_limit) physical_ram = std::min(physical_ram, cgroup_limit);
   

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   brunoerg commented at 7:02 pm on March 6, 2026:

   style nit (for consistency between cgroup v1 and v2):

    0diff --git a/src/common/system.cpp b/src/common/system.cpp
    1index 862bcd8a7e..6f309b3797 100644
    2--- a/src/common/system.cpp
    3+++ b/src/common/system.cpp
    4@@ -139,8 +139,10 @@ std::optional<size_t> GetTotalRAM()
    5                 if (auto limit{ToIntegral<uint64_t>(val)}) physical_ram = std::min(physical_ram, *limit);
    6             }
    7         } else if (std::ifstream cg1{"/sys/fs/cgroup/memory/memory.limit_in_bytes"}; cg1) {
    8-            uint64_t cgroup_limit;
    9-            if (cg1 >> cgroup_limit) physical_ram = std::min(physical_ram, cgroup_limit);
   10+            std::string val;
   11+            if (cg1 >> val) {
   12+                if (auto limit{ToIntegral<uint64_t>(val)}) physical_ram = std::min(physical_ram, *limit);
   13+            }
   14         }
   15 #endif // __linux__
   16         return clamp(physical_ram);
   

6. ViniciusCestarii force-pushed on Mar 6, 2026
7. 
   l0rinc commented at 8:32 pm on March 6, 2026: contributor

   This was discussed before in #34641 (review)
8. 
   ViniciusCestarii commented at 10:41 am on March 7, 2026: none

   I wasn’t aware of this. I agree that making it container-aware may not be worth it, so I’ll close the PR.

   It might still be useful to add a short comment in the code explaining that this behavior is intentional and not meant to be container-aware, to avoid future confusion.

9. ViniciusCestarii closed this on Mar 7, 2026

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Contributors
ViniciusCestarii DrahtBot brunoerg l0rinc