perftune: tune tcp_mem

tcp_mem defaults to 9% of memory, while Seastar defaults to 93% of memory.
These sum to 102% which leads to OOM.

Fix by tuning tcp_mem, by default to 3% of memory. The low/pressure/max
ratios are the same as those selected by the kernel.

Author: avikivity

Diff for: scripts/perftune.py

@@ -606,6 +606,17 @@ def tune(self):
         # did not receive an acknowledgment from connecting client.
         fwriteln_and_log('/proc/sys/net/ipv4/tcp_max_syn_backlog', '4096')
 
+        self.tune_tcp_mem()
+
+    def tune_tcp_mem(self):
+        page_size = os.sysconf("SC_PAGE_SIZE")
+        phys_mem = os.sysconf("SC_PHYS_PAGES") * page_size
+        # We only tune for physical memory since tcp_mem is virtualized
+        def to_pages(bytes):
+            return math.ceil(bytes / page_size)
+        max = phys_mem * self.args.tcp_mem_fraction
+        fwriteln_and_log('/proc/sys/net/ipv4/tcp_mem', f"{to_pages(max / 2)} {to_pages(max * 2/3)} {to_pages(max)}")
+
     def nic_is_bond_iface(self, nic):
         return self.__nic_is_bond_iface.get(nic, False)
 
@@ -1627,6 +1638,7 @@ def names():
                                                           "CPU cores out of available according to a 'cpu_mask' value."
                                                           "Default is 16",
                   type=int, default=16, dest='cores_per_irq_core')
+argp.add_argument('--tcp-mem-fraction', default=0.03, type=float, help="Fraction of total memory to allocate for TCP buffers")
 
 def parse_cpu_mask_from_yaml(y, field_name, fname):
     hex_32bit_pattern='0x[0-9a-fA-F]{1,8}'