[Pvfs2-developers] the halloween bug fixed

[Sam Lang]

The halloween bug (what I'm calling it -- its been haunting us for a  
while now) is that we're adding address references to the bmi address  
list, and never removing them.  In the prelude state machine, we make  
a BMI_set_info(addr, BMI_INC_ADDR_REF) call, which iterates through  
all the addresses in the reference list.  Its this step that is  
causing the slowdown.  As new connections are made addr refs get  
added to the list and never removed, so the pvfs2-client-core addr  
ref ends up at the bottom of a very long list.

The addr refs aren't getting removed, because in BMI_set_info(addr,  
BMI_DEC_ADDR_REF)  -- called from final_response -- the code queries  
the bmi_tcp method on whether the address should be removed  
BMI_tcp_get_info(BMI_DROP_ADDR_QUERY).  This function always returns  
false (don't drop), unless there was a bmi error somewhere (ECANCEL  
is probably the only one that happens in practice -- due to a timeout).

Since our state actions block the main server thread, this caused  
degradation for all requests received during processing of requests  
from a long-lived socket.  New connections hitting the server at  
different times would have been fine though, which is what I was  
seeing with my tests.

The obvious and easy fix is to have bmi-tcp return true from  
DROP_ADDR_QUERY for all address references.  As far as I can tell,  
the only thing we save by keeping them around is a little memory  
allocation (the socket gets closed either way).

In the changes I've been working on to get multiple address support  
in BMI, I've already replaced the linked list with a hashtable, which  
wouldn't have made the problem go away, but the degradation wouldn't  
have been quite as bad (may have made it harder to find, actually).   
Maybe its time to add some profiling info (perf stats?) to our basic  
list, queue and hash structures that would tell us how big they're  
getting.

Anyway, thanks to all for contributing to the debugging process for  
this one.

-sam

On Sep 26, 2007, at 6:00 PM, Sam Lang wrote:

>
> Hi All,
>
> I've been trying to debug a problem with PVFS, where performance  
> degrades slowly with a long-lived (weeks and months) PVFS volume.   
> The degradation is significant -- simple metadata operations are an  
> order of magnitude slower after a month or so.  The behavior turns  
> out to only occur with the VFS and pvfs2-client daemon:   
> performance of the admin tools (pvfs2-touch, pvfs2-rm, etc.) to the  
> same set of servers remains good.  Restarting the client daemon  
> also fixes the problem, suggesting that the long-lived open sockets  
> are somehow the cause.  The slowness also appears to be at the  
> servers not the clients: the same kernel module and client daemon  
> to a different filesystem and set of servers doesn't exhibit the  
> performance degradation.
>
> Also, I should mention that the system config is a little different  
> than usual.  We have IO nodes mounting and unmounting the PVFS  
> volume  (and stopping the client daemon) with each user's job,  
> which is fairly frequent, while on the login nodes, the volume  
> remains mounted for a long time (and where the performance degrades).
>
> Our hunch here is that epoll or our use of epoll on the servers is  
> somehow to blame.  Maybe the file descriptors opened on the server  
> for pvfs2-client-core are getting pushed down further and further  
> into the epoll set, which for some reason is growing with new  
> connections coming and going.  This might be the case if we were  
> failing to remove sockets from the set on disconnect, for example.   
> It doesn't look like that's happening though, at least for normal  
> disconnects.
>
> Its a PITA to debug, because the servers have to remain running for  
> a long time (and the clients have to remain mounted) for the  
> problem to be visible.  Rob suggested I use strace on the servers  
> to see what epoll was doing, and that showed some interesting  
> results.  Basically, it looks like epoll_wait takes significantly  
> longer when clients are doing operations over the VFS, rather than  
> with the pvfs2 admin tools.  Also, strace reported epoll_ctl(...,  
> EPOLL_CTL_ADD, ...)) getting called a few times, even for the VFS  
> ops, and in those cases its returning EEXISTS.
>
> I noticed that we add a socket to the epoll set whenever we get a  
> new connection, or a read or write is posted (enqueue_operation),  
> but we only remove the socket from the epoll set on errors or  
> disconnects.  So why are we adding it for reads and writes?  Any  
> connected socket should already be in the set, no?  I think this  
> may be why I'm seeing EEXISTS with strace.
>
> Also, is it safe to check the error from epoll_ctl in  
> BMI_socket_collection_[add|remove]?
>
> And finally, assuming PVFS is actually using epoll calls properly,  
> does anyone know of epoll bugs on a SUSE 2.6.5 kernel that would  
> cause epoll_ctl(..., EPOLL_CTL_DEL, ....) to not do what its meant  
> to?  Googling epoll and SUSE 2.6.5 isn't turning up anything...
>
> Thanks,
> -sam
>