[Gluster-devel] [RFC ] dictionary optimizations

Xavier Hernandez xhernandez at datalab.es
Fri Sep 6 08:46:23 UTC 2013 

Al 04/09/13 18:10, En/na Anand Avati ha escrit:
> On Wed, Sep 4, 2013 at 6:37 AM, Xavier Hernandez 
> <xhernandez at datalab.es <mailto:xhernandez at datalab.es>> wrote:
>
>     Al 04/09/13 14:05, En/na Jeff Darcy ha escrit:
>
>         On 09/04/2013 04:27 AM, Xavier Hernandez wrote:
>
>             I would also like to note that each node can store
>             multiple elements.
>             Current implementation creates a node for each byte in the
>             key. In my
>             implementation I only create a node if there is a prefix
>             coincidence between
>             2 or more keys. This reduces the number of nodes and the
>             number of
>             indirections.
>
>
>         Whatever we do, we should try to make sure that the changes
>         are profiled
>         against real usage.  When I was making my own dict
>         optimizations back in March
>         of last year, I started by looking at how they're actually
>         used. At that time,
>         a significant majority of dictionaries contained just one
>         item. That's why I
>         only implemented a simple mechanism to pre-allocate the first
>         data_pair instead
>         of doing something more ambitious.  Even then, the difference
>         in actual
>         performance or CPU usage was barely measurable.  Dict usage
>         has certainly
>         changed since then, but I think you'd still be hard pressed to
>         find a case
>         where a single dict contains more than a handful of entries,
>         and approaches
>         that are optimized for dozens to hundreds might well perform
>         worse than simple
>         ones (e.g. because of cache aliasing or branch misprediction).
>
>         If you're looking for other optimization opportunities that
>         might provide even
>         bigger "bang for the buck" then I suggest that stack-frame or
>         frame->local
>         allocations are a good place to start.  Or string copying in
>         places like
>         loc_copy.  Or the entire fd_ctx/inode_ctx subsystem.  Let me
>         know and I'll come
>         up with a few more.  To put a bit of a positive spin on
>         things, the GlusterFS
>         code offers many opportunities for improvement in terms of CPU
>         and memory
>         efficiency (though it's surprisingly still way better than
>         Ceph in that regard).
>
>     Yes. The optimizations on dictionary structures are not a big
>     improvement in the overall performance of GlusterFS. I tried it on
>     a real situation and the benefit was only marginal. However I
>     didn't test new features like an atomic lookup and remove if found
>     (because I would have had to review all the code). I think this
>     kind of functionalities could improve a bit more the results I
>     obtained.
>
>     However this is not the only reason to do these changes. While
>     I've been writing code I've found that it's tedious to do some
>     things just because there isn't such functions in dict_t. Some
>     actions require multiple calls, having to check multiple errors
>     and adding complexity and limiting readability of the code. Many
>     of these situations could be solved using functions similar to
>     what I proposed.
>
>     On the other side, if dict_t must be truly considered a concurrent
>     structure, there are a lot of race conditions that might appear
>     when doing some operations. It would require a great effort to
>     take care of all these possibilities everywhere. It would be
>     better to pack most of these situations into functions inside the
>     dict_t itself where it is easier to combine some operations.
>
>     By the way, I've made some tests with multiple bricks and it seems
>     that there is a clear speed loss on directory listings as the
>     number of bricks increases. Since bricks should be independent and
>     they can work in parallel, I didn't expected such a big
>     performance degradation.
>
>
> The likely reason is that, even though bricks are parallel for IO, 
> readdir is essentially a sequential operation and DHT has a limitation 
> that a readdir reply batch does not cross server boundaries. So if you 
> have 10 files and 1 server, all 10 entries are returned in one call to 
> the app/libc. If you have 10 files and 10 servers evenly distributed, 
> the app/libc has to perform 10 calls and keeps getting one file at a 
> time. This problem goes away when each server has enough files to fill 
> up a readdir batch. It's only when you have too few files and too many 
> servers that this "dilution" problem shows up. However, this is just a 
> theory and your problem may be something else too..
>
I didn't know that DHT was doing a sequential brick scan on readdir(p) 
(my fault). Why is that ? Why it cannot return entries crossing a server 
boundary ? is it due to a technical reason or is it only due to the 
current implementation ?

I've made a test using only directories (50 directories with 50 
subdirectories each). I started with one brick and I measured the time 
to do a recursive 'ls'. Then I sequentially added an additional brick, 
up to 6 (all of them physically independent), and repeated the ls. The 
time increases linearly as the number of bricks augments. As more bricks 
were added, the rebalancing time was also growing linearly.

I think this is a big problem for scalability. It can be partially 
hidden by using some caching or preloading mechanisms, but it will be 
there and it will hit sooner or later.

> Note that Brian Foster's readdir-ahead patch should address this 
> problem to a large extent. When loaded on top of DHT, the prefiller 
> effectively collapses the smaller chunks returned by DHT into a larger 
> chunk requested by the app/libc.
>
I've seen it, however I think it will only partially mitigate and hide 
an existing problem. Imagine you have some hundreds or a thousand of 
bricks. I doubt readdir-ahead or anything else can hide the enormous 
latency that the sequential DHT scan will generate in that case.

The main problem I see is that the full directory structure is read many 
times sequentially. I think it would be better to do the readdir(p) 
calls in parallel and combine them (possibly in background). This way 
the time to scan the directory structure would be almost constant, 
independently of the number of bricks.

Xavi

> Avati
>
>
>     However the tests have not been exhaustive nor made in best
>     conditions so they might be misleading. Anyway it seems to me that
>     there might be a problem with some mutexes that force too much
>     serialization of requests (though I have no real proves it's only
>     a feeling). Maybe some more "asynchronousity" on calls between
>     translators could help.
>
>     Only some thoughts...
>
>     Best regards,
>
>     Xavi
>
>
>
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