diff --git a/pbs-tools/src/lib.rs b/pbs-tools/src/lib.rs
index 053b3c0c..939ba7e6 100644
--- a/pbs-tools/src/lib.rs
+++ b/pbs-tools/src/lib.rs
@@ -8,3 +8,22 @@ pub mod sha;
 pub mod ticket;
 
 pub mod async_lru_cache;
+
+/// Set MMAP_THRESHOLD to a fixed value (128 KiB)
+///
+/// This avoids the "dynamic" mmap-treshold logic from glibc's malloc, which seems misguided and
+/// effectively avoids using mmap for all allocations smaller than 32 MiB. Which, in combination
+/// with the allocation pattern from our/tokio's complex async machinery, resulted in very large
+/// RSS sizes due to defragmentation and long-living (smaller) allocation on top of the heap
+/// avoiding that the (big) now free'd allocations below couldn't get given back to the OS. This is
+/// not an issue with mmap'd memeory chunks, those can be given back at any time.
+///
+/// Lowering effective MMAP treshold to 128 KiB allows freeing up memory to the OS better and with
+/// lower latency, which reduces the peak *and* average RSS size by an order of magnitude when
+/// running backup jobs. We measured a reduction by a factor of 10-20 in experiments and see much
+/// less erratic behavior in the overall's runtime RSS size.
+pub fn setup_libc_malloc_opts() {
+    unsafe {
+        libc::mallopt(libc::M_MMAP_THRESHOLD, 4096*32);
+    }
+}
diff --git a/proxmox-backup-client/src/main.rs b/proxmox-backup-client/src/main.rs
index 50682637..406f2d18 100644
--- a/proxmox-backup-client/src/main.rs
+++ b/proxmox-backup-client/src/main.rs
@@ -1464,6 +1464,7 @@ impl ReadAt for BufferedDynamicReadAt {
 }
 
 fn main() {
+    pbs_tools::setup_libc_malloc_opts();
 
     let backup_cmd_def = CliCommand::new(&API_METHOD_CREATE_BACKUP)
         .arg_param(&["backupspec"])
diff --git a/proxmox-restore-daemon/src/main.rs b/proxmox-restore-daemon/src/main.rs
index c2be942b..47790a7d 100644
--- a/proxmox-restore-daemon/src/main.rs
+++ b/proxmox-restore-daemon/src/main.rs
@@ -38,6 +38,8 @@ lazy_static! {
 
 /// This is expected to be run by 'proxmox-file-restore' within a mini-VM
 fn main() -> Result<(), Error> {
+    pbs_tools::setup_libc_malloc_opts();
+
     if !Path::new(VM_DETECT_FILE).exists() {
         bail!(
             "This binary is not supposed to be run manually, use 'proxmox-file-restore' instead."
diff --git a/src/bin/proxmox-backup-api.rs b/src/bin/proxmox-backup-api.rs
index e6fc5f23..ee037a3b 100644
--- a/src/bin/proxmox-backup-api.rs
+++ b/src/bin/proxmox-backup-api.rs
@@ -19,6 +19,8 @@ use proxmox_backup::auth_helpers::*;
 use proxmox_backup::config;
 
 fn main() {
+    pbs_tools::setup_libc_malloc_opts();
+
     proxmox_backup::tools::setup_safe_path_env();
 
     if let Err(err) = proxmox_async::runtime::main(run()) {
diff --git a/src/bin/proxmox-backup-proxy.rs b/src/bin/proxmox-backup-proxy.rs
index 523966cf..30b730ef 100644
--- a/src/bin/proxmox-backup-proxy.rs
+++ b/src/bin/proxmox-backup-proxy.rs
@@ -73,6 +73,8 @@ use proxmox_backup::server::do_verification_job;
 use proxmox_backup::server::do_prune_job;
 
 fn main() -> Result<(), Error> {
+    pbs_tools::setup_libc_malloc_opts();
+
     proxmox_backup::tools::setup_safe_path_env();
 
     let backup_uid = pbs_config::backup_user()?.uid;