Hello Maloy,
On Thu, Jun 3, 2021 at 3:50 AM Jon Maloy [off-list ref] 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
On 6/2/21 10:26 PM, Menglong Dong wrote:
Hello Maloy,
On Thu, Jun 3, 2021 at 3:50 AM Jon Maloy [off-list ref] wrote:
[...]
quoted
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?
Yes, I think that makes sense. I was always aware of the "fragility" of
my approach, -this one looks more future safe.
///jon
Thanks
Menglong Dong
On Thu, Jun 3, 2021 at 9:08 PM Jon Maloy [off-list ref] wrote:
[...]
Yes, I think that makes sense. I was always aware of the "fragility" of
my approach, -this one looks more future safe.
Ok, I will post a patch.
Menglong Dong