[Pvfs2-developers] Re: the halloween bug fixed

[Sam Lang]

On Oct 5, 2007, at 10:49 AM, Sam Lang wrote:

>
> 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).

This suggested fix isn't right.  The DEC_ADDR_REF which decrements  
the refcount to zero, is invoked after sending the final response,  
but that's usually before the client (in the case of the admin tools)  
closes the connection.  It looks like its the tcp_forget_addr in the  
bmi method that needs to call back out to the bmi wrapper layer to  
remove the reference from the list.  I can call BMI_set_info(addr,  
BMI_TCP_CLOSE_SOCKET) from tcp_forget_addr, but that seems a bit  
backwards...

-sam

>
> 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
>>
>