Re: ext3 allocate-with-reservation latencies

[Stephen C. Tweedie]

Hi,

On Fri, 2005-04-08 at 19:10, Mingming Cao wrote:

> > It still needs to be done under locking to prevent us from expanding
> > over the next window, though.  And having to take and drop a spinlock a
> > dozen times or more just to find out that there are no usable free
> > blocks in the current block group is still expensive, even if we're not
> > actually fully unlinking the window each time.

> Isn't this a rare case? The whole group is relatively full and the free
> bits are all reserved by other files.

Well, that's not much different from the case where there _are_ usable
bits but they are all near the end of the bitmap.  And that's common
enough as you fill up a block group with small files, for example.  Once
the bg becomes nearly full, new file opens-for-write will still try to
allocate in that bg (because that's where the inode was allocated), but
as it's a new fd we have no prior knowledge of _where_ in the bh to
allocate, and we'll have to walk it to the end to find any free space. 
This is the access pattern I'd expect of (for example) "tar xvjf
linux-2.6.12.tar.bz2", not exactly a rare case.

>   Probably we should avoid trying
> to make reservation in this block group at the first place

Not in this case, because it's the "home" of the file in question, and
skipping to another bg would just leave useful space empty --- and that
leads to fragmentation.

> You are proposing that we hold the read lock first, do the window search
> and bitmap scan, then once we confirm there is free bit in the candidate
> window, we grab the write lock and update the tree?  

No, I'm suggesting that if we need the write lock for tree updates, we
may still be able to get away with just a read lock when updating an
individual window.  If all we're doing is winding the window forwards a
bit, that's not actually changing the structure of the tree.

> However I am still worried that the rw lock will allow concurrent files
> trying to lock the same window at the same time. 

Adding a new window will need the write lock, always.  But with the read
lock, we can still check the neighbouring windows (the structure is
pinned so those remain valid), so advancing a window with just that lock
can still be done without risking overlapping the next window.

--Stephen

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