Messages in this thread | | | Subject | Re: The value of FB_MTU eats two pages | From | Jon Maloy <> | Date | Wed, 2 Jun 2021 15:50:27 -0400 |
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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
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