[PATCH v3 10/13] builtin/gc.c: correct RAM calculation when using sysinfo

From: Ramsay Jones <ramsay@ramsayjones.plus.com>
Date: Apr 16, 2025, 23:18 UTC
Message-ID: <20250416231835.2492562-11-ramsay@ramsayjones.plus.com>
In-Reply-To: <20250416231835.2492562-1-ramsay@ramsayjones.plus.com>

The man page for sysinfo(2) on Linux states that (from v2.3.48) the sizes of the memory and swap fields, of the returned structure, are given as multiples of 'mem_unit' bytes. In earlier versions (prior to v2.3.23 on i386 in particular), the 'mem_unit' field was not part of the structure, and all sizes were measured in bytes. The man page does not discuss the motivation for this change, but it is possible that the change was intended for the, relatively rare, 32-bit platform with more than 4GB of memory.

The total_ram() function makes the assumption that the 'totalram' field of the 'struct sysinfo' is measured in bytes, or alternatively that the 'mem_unit' field is always equal to one. Having writen a program to call the sysinfo() function and print the structure fields, it seems that, on Linux x84_64 and i686 anyway, the 'mem_unit' field is indeed set to one (note that the 32-bit system had only 2GB ram). However, cygwin also has an sysinfo() implementation, which gives the following values:

  $ ./sysinfo
  uptime:      21381
  loads:       0, 0, 0
  total ram:   2074637
  free ram:    843237
  shared ram:  0
  buffer ram:  0
  total swap:  327680
  free swap:   306932
  procs:       15
  total high:  0
  free high:   0
  mem_unit:    4096

  total ram: 8497713152
  $

[This laptop has 8GB ram, so a little bit seems to be missing. ;) ]

Modify the total_ram() function to allow for the possibility that the memory size is not specified in bytes (ie 'mem_unit' is greater than one).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
---
 builtin/gc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 99431fd467..cdcf1dc6e7 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -373,8 +373,13 @@ static uint64_t total_ram(void)
 #if defined(HAVE_SYSINFO)
 	struct sysinfo si;
 
-	if (!sysinfo(&si))
-		return si.totalram;
+	if (!sysinfo(&si)) {
+		uint64_t total = si.totalram;
+
+		if (si.mem_unit > 1)
+			total *= (uint64_t)si.mem_unit;
+		return total;
+	}
 #elif defined(HAVE_BSD_SYSCTL) && (defined(HW_MEMSIZE) || defined(HW_PHYSMEM))
 	int64_t physical_memory;
 	int mib[2];

-- 
2.49.0