Messages in this thread | | | From | Menglong Dong <> | Date | Thu, 3 Jun 2021 10:26:55 +0800 | Subject | Re: The value of FB_MTU eats two pages |
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Hello Maloy,
On Thu, Jun 3, 2021 at 3:50 AM Jon Maloy <jmaloy@redhat.com> wrote:
[...] > 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.
I'm not sure if this is a perfect way to determine the value of FB_MTU. If 'struct skb_shared_info' changes, this value seems should change, too.
How about we make it this:
#define FB_MTU (PAGE_SIZE - \ SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) - \ SKB_DATA_ALIGN(BUF_HEADROOM + BUF_TAILROOM + 3 + \ MAX_H_SIZ))
The value 'BUF_HEADROOM + BUF_TAILROOM + 3' come from 'tipc_buf_acquire()':
#ifdef CONFIG_TIPC_CRYPTO unsigned int buf_size = (BUF_HEADROOM + size + BUF_TAILROOM + 3) & ~3u; #else unsigned int buf_size = (BUF_HEADROOM + size + 3) & ~3u; #endif
Is it a good idea?
Thanks Menglong Dong
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