RE: [PATCH 5.4 175/244] inet: use bigger hash table for IP ID generation

From: Amit Klein

Sent: 16 June 2021 10:17

...

-#define IP_IDENTS_SZ 2048u

+/* Hash tables of size 2048..262144 depending on RAM size.

• • Each bucket uses 8 bytes.
• */

+static u32 ip_idents_mask __read_mostly;

...

• /* For modern hosts, this will use 2 MB of memory */
• idents_hash = alloc_large_system_hash("IP idents",

•                      sizeof(*ip_idents) + sizeof(*ip_tstamps),
  

•                      0,
  

•                      16, /* one bucket per 64 KB */
  

•                      HASH_ZERO,
  

•                      NULL,
  

•                      &ip_idents_mask,
  

•                      2048,
  

•                      256*1024);
  

Can someone explain why this is a good idea for a 'normal' system?

Why should my desktop system 'waste' 2MB of memory on a massive hash table that I don't need. It might be needed by systems than handle massive numbers of concurrent connections - but that isn't 'most systems'.

Surely it would be better to detect when the number of entries is comparable to the table size and then resize the table.

David

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