Hello
On Fri, Apr 8, 2016 at 7:58 PM, Shuhrat Rahimov <
sandrorkmv@gmail.com>
wrote:
> Hello,
> I was reading this post (
> http://www.yaffs.net/lurker/message/20090812.040040.5e77b55f.en.html )
> regarding the tnode size and RAM usage by the tnodes and have been confused.
> When we look at source code at yaffs_guts_initialise() where the tnode
> size is set we can see:
>
> dev->tnode_size = (dev->tnode_width * YAFFS_NTNODES_LEVEL0) / 8;
> if (dev->tnode_size < sizeof(struct yaffs_tnode))
> dev->tnode_size = sizeof(struct yaffs_tnode);
> So, in this case in ARM sizeof pointer would be 4 B (in 64-bit system even
> greater) which gives sizeof(struct yaffs_tnode) = 32 B. In
> yaffs_allocator.c in function yaffs_create_tnodes() 100 tnodes are
> allocated at a time and there the memory allocation size is = n_tnodes *
> dev->tnode_size = 100*32 = 3.2K for only 100 tnodes (here, according to the
> post above I would expect 100*20 bit = 250
>
Byte of RAM). In the post above it was stated that for 524288 chunks it was
> needed 20 bits to represent each of them, but in code we can see that it is
> needed at least 32 B = 26 bits for each tnode irrespective of the bits
> needed to represent each tnode. Could you explain it to me, how much memory
> is needed for a tnode? Would it be possible to decrease RAM usage by
> decreasing YAFFS_NTNODES_LEVEL0 and other relevant constants or is tnode
> size irrespective of that always at least sizeof(struct yaffs_tnode)?
>
The bit you are missing is that each the tnode structure holds multiple
tnodes.
For level 0 Tnodes that is 16 tnodes. For internal (level 1,2,...) it is 8
tnodes.
You can adjust the size of the TNODE structure by adjusting:
#define YAFFS_NTNODES_LEVEL0 16
#define YAFFS_TNODES_LEVEL0_BITS 4
#define YAFFS_TNODES_LEVEL0_MASK 0xf
#define YAFFS_NTNODES_INTERNAL (YAFFS_NTNODES_LEVEL0 / 2)
#define YAFFS_TNODES_INTERNAL_BITS (YAFFS_TNODES_LEVEL0_BITS - 1)
#define YAFFS_TNODES_INTERNAL_MASK 0x7
Have you read HowYaffsWorks? That helps to explain some of this better:
http://www.yaffs.net/sites/yaffs.net/files/yaffs.net/HowYaffsWorks.pdf
Regards
Charles
> Regards,
> Shuhrat Rahimov
>
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