Re: The value of FB_MTU eats two pages

From: Jon Maloy
Date: Wed Jun 02 2021 - 15:50:41 EST




On 6/1/21 10:18 AM, Menglong Dong wrote:
Hello!

I have a question about the value of FB_MTU in tipc, how does the '3744' form?
I notice that it is used in 'tipc_msg_build()' when memory allocation
fails, and it
tries to fall back to a smaller MTU to avoid unnecessary sending failures.

However, the size of the data allocated will be more than 4096 when FB_MTU
is 3744. I did a rough calculation, the size of data will more than 4200:

(FB_MTU + TIPCHDR + BUF_HEADROOM + sizeof(struct skb_shared_info))

Therefore, 8192 will be allocated from slab, and about 4000 of it will
not be used.

FB_MTU is used for low memory, and I think eating two pages will make it worse.
Do I miss something?

Thanks!
Menglong Dong

Hi Dong,
The value is based on empiric knowledge.
When I determined it I made a small loop in a kernel driver where I allocated skbs (using tipc_buf_acquire) with an increasing size (incremented with 1 each iteration), and then printed out the corresponding truesize.

That gave the value we are using now.

Now, when re-running the test I get a different value, so something has obviously changed since then.

[ 1622.158586] skb(513) =>> truesize 2304, prev skb(512) => prev truesize 1280
[ 1622.162074] skb(1537) =>> truesize 4352, prev skb(1536) => prev truesize 2304
[ 1622.165984] skb(3585) =>> truesize 8448, prev skb(3584) => prev truesize 4352

As you can see, the optimal value now, for an x86_64 machine compiled with gcc, is 3584 bytes, not 3744.

Feel free to post a patch for this if you want to.

Thanks
///Jon Maloy