Charles Manning wrote:
> On Wednesday 12 August 2009 16:00:40 Andrew McKay wrote:
>> Hey Charles,
>>
>> When testing with the 2GB NAND I was visited by the OOM killer a few times.
>>  This makes me think we're short on RAM for handling a 2GB NAND part.  Our
>> board currently has 32MB of RAM, of which 8MB is used for a RAM disk.  When
>> I dropped the ramdisk down to 3.5MB for testing purposes, I didn't have
>> issues with the OOM killer any more.  We're looking at moving up to 64MB of
>> RAM to avoid this issue.  However in the future I'd like to be able to
>> estimate the memory usage of YAFFS2 based on NAND size.
>>
>> I found a thread about YAFFS2 memory usage, and I just want to make sure I
>> understand it correctly.
>>
>> http://www.yaffs.net/lurker/message/20090701.190059.23524635.ca.html#yaffs
>>
>>> * yaffs_Objects: Each object (file, directory,...) holds a yaffs_Object
>> in memory which is around 120 bytes per object.
>>
>> So every file, directory, etc. uses up 120 bytes of RAM.  This is all the
>> time? Right from when the filesystem is mounted?  So if I have 1000 objects
>> on the device I'll be using up 120000 bytes?
>>
>>> * yaffs_Tnodes: These are the things used to build the trees.
>> The part I'm using is 8192 erase blocks, and 64 pages per erase block.
>> That means there are 524288 chunks in my filesystem.  Using your equation I
>> come up with
>>
>> Log2(524288) = 19 bits
>> 19 + 1 = 20 (which is already even)
>>
>> So 20 bits will be used to represent each chunk.
>>
>> Assuming worst case and the filesystem is full I will be using all 524288
>> chunks.  This means that I will need 20 * 512K which is 10MB of RAM to
>> store all the Tnodes.  Does that seem about right?
> 
> Bits mate, not bytes... Therefore that calc should come out to be closer to 
> 1.2Mbytes
> 
> The calculation is somewhat complicated by the fact that Tnodes are managed in 
> arrays of 16, ie 40 bytes in this case. Thus:
> * On average larger files will have a wasted 20 bytes of Tnode.
> * Very small files will still need 40 bytes of Tnode, even if they only use a 
> small amount of that.
> 
> Thus lots of small files will skew the numbers a bit.
> 
> If you do a /proc/yaffs you can get the actual numbers in use and use those
> 
> nTnodesCreated * 40 bytes
> +
> nObjectsCreated * approx 120 bytes
> 
> It would be possible to make a tweak to handle very short files better. Files 
> smaller than 1 chunk don't really need a tnode tree since the tnode pointer 
> could be stored directly in the object structure.

Base on your experience do you have a rule of thumb for amount of RAM required 
based on the size or number of chunks of NAND?

Thanks,
Andrew McKay
Iders Inc.