David Miller a écrit :
From: Eric Dumazet Hi <[EMAIL PROTECTED]>
Date: Tue, 17 Oct 2006 14:58:37 +0200
reqsk_queue_alloc() goal is to use a power of two size for the whole
listen_sock structure, to avoid wasting memory for large backlogs,
meaning the hash table nr_table_entries is not anymore a power of
two. (Hence one AND (nr_table_entries - 1) must be replaced by
MODULO nr_table_entries)
Modulus can be very expensive for some small/slow cpus. Please round
down to a power-of-2 instead of up if you think the wastage really
matters.
Thanks.
I am not sure I understand your points. Rounding up or down still need the
modulus. Only the size changes by a two factor. I feel you want me to remove
the modulus, thats unrelated to rounding.
A 66 MHz 486 can perform 1.000.000 divisions per second. Is it a 'slow' cpu ?
If we stay with a power-of-two, say 2^X hash slots, using (2^X)*sizeof(void*),
the extra bits added by struct listen_sock will *need* the same amount of
memory, because of kmalloc() alignment to next power-of-two. That basically
wastes half of the ram taken by struct listen_sock allocation, unless we add
yet another pointer to hash table and do two kmallocs(), one for pure
power-of-two hash table, one for struct listen_sock. If we keep current
scheme, the current max kmalloc size of 131072 bytes would limit us to 65536
bytes for the hash table itself, so 8192 slots on 64bits platforms. I was
expecting to use a 16380 slots hash size instead.
The modulus is done on two places :
inet_csk_search_req() : called from tcp_v4_err()/dccp_v4_err() only after
checks. Frequency of such events is rather low.
tcp_v4_hnd_req() : called from tcp_v4_do_rcv() for TCP_LISTEN state. Frequency
of such events is rather low, especially on machines driven by small/slow cpus...
inet_csk_reqsk_queue_hash_add()called from tcp_v4_conn_request() when a new
connection attempt is stored in hash table.
Thats in normal conditions two modulus done per new tcp/dccp sessions
establishments. In DOS situation, I doubt the extra cycles will do any difference.
So... what do you prefer :
1) Keep the modulus
2) allocate two blocks of ram (powser-of -two hash size, but one extra
indirection)
3) waste near half of ram because one block allocated, and power-of-two hash
size.
Thank you
Eric
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