[Devel] Re: [RFC v11][PATCH 03/13] General infrastructure for checkpoint restart

[Oren Laadan]

Mike Waychison wrote:
> Oren Laadan wrote:
>>
>> Dave Hansen wrote:
>>> On Tue, 2008-12-16 at 13:54 -0800, Mike Waychison wrote:
>>>> Oren Laadan wrote:
>>>>> diff --git a/checkpoint/sys.c b/checkpoint/sys.c
>>>>> index 375129c..bd14ef9 100644
>>>>> --- a/checkpoint/sys.c
>>>>> +++ b/checkpoint/sys.c
>>>>> +/*
>>>>> + * During checkpoint and restart the code writes outs/reads in data
>>>>> + * to/from the checkpoint image from/to a temporary buffer
>>>>> (ctx->hbuf).
>>>>> + * Because operations can be nested, use cr_hbuf_get() to reserve
>>>>> space
>>>>> + * in the buffer, then cr_hbuf_put() when you no longer need that
>>>>> space.
>>>>> + */
>>>> This seems a bit over-kill for buffer management no?  The only large
>>>> header seems to be cr_hdr_head and the blowup comes from utsinfo
>>>> string data (which could easily be moved out to be in it's own
>>>> CR_HDR_STRING blocks).
>>>>
>>>> Wouldn't it be easier to use stack-local storage than balancing the
>>>> cr_hbuf_get/put routines?
>>> I've asked the same question, so I'll give you Oren's response that I
>>> remember:
>>>
>>> cr_hbuf_get/put() are more of an API that we can use later.  For now,
>>> those buffers really are temporary.  But, in a case where we want to do
>>> a really fast checkpoint (to reduce "downtime" during the checkpoint) we
>>> store the image entirely in kernel memory to be written out later.
>>
>> Precisely.
>>
>> Note that by "store the image entirely" we mean everything that cannot
>> be saved COW - so memory pages are not duplicated; the rest of the data
>> tends to take less than 5-10% of the total size.
>>
>> Buffering the checkpoint image in kernel is useful to reduce downtime
>> during checkpoint, and also useful for super-fast rollback of a task
>> (or container) by always keeping everything in memory.
> 
> I agree that buffering metadata and references to COWable structures is
> useful, but this doesn't require a new allocator.

First, many thanks for the comments !

The idea is to hide the details behind a pair of operation: one to get a
buffer, and the other to "deposit" the data (in non-buffering mode this
means write out and free).
My first goal is to have all code use a standard method (cr_hbuf_get/put);
the specific implementation of which may vary for buffering/non-buffering
cases, and may change.
(In the code, I used a "static" common buffer attached to 'struct ct_ctx'
for simplicity and speed, and because it is similar in idea to how the
buffering mode could be implemented).

> 
>>
>> This abstraction is also useful for restart, e.g. to implement read-ahead
>> of the checkpoint image into the kernel.
> 
> I'm not sure what you mean by checkpoint image read-ahead.  The data is
> in a stream format and not a randomly accessible packet format.

I mean that for efficiency, rather than reading many small chunks of data
(header by header), we could read-ahead a large buffer into kernel memory
and then have cr_hbuf_get() provide a pointer to the right position, and
cr_hbuf_put() do nothing.

>>
>> Note also that in the future we will have larger headers (e.g. to record
>> the state of a socket), 
> 
> This is a bit off-topic for this patch series, but what are your intents
> on socket serialization?

I should have been more precise - I was referring to unix domain sockets.

> 
> FWIW looking at the socket problem for our purposes, we expect to throw
> away all network sockets (and internal state including queued rx/tx
> data) at checkpoint (replacing them with sockets that return ECONNRESET
> at restore time) and rely on userland exception handling to re-establish
> RPC channels.  This failure mode looks exactly the same as a network
> partition/machine restart/application crash which our applications need
> to handle already.

Yes.

But ... we also have to support non-TCP and general connection-less
sockets too.

Also, there is the case of INET connections that are confined to the
container - connections to localhost (or local IP): since they do not
depend on the outside world, we should restore them as is.

Finally, in the case of live migration, where you wanna migrate the
network connections too, and then there is quite a bit of state associated.
But that it far down the road...

> 
>> and there may be some nested calls (e.g. to dump
>> a connected unix-domain socket we will want to first save the "parent"
>> listening socket, and also there is nesting in restart).
>>
> 
> Right, nesting can be a problem, so maybe on stack isn't the best way to
> handle the header allocations, but again this doesn't necessarily mean
> we need a new object allocation scheme.
> 
>> Instead of using the stack for some headers and memory allocation for
>> other headers, this abstraction provides a standard interface for all
>> checkpoint/restart code (and the actual implementation may vary for
>> different purposes).
> 
> At a minimum, I would expect that cr_hbuf_put() should take a pointer
> rather than a size argument.

I agree that this is nicer in terms of API. However, requiring the
size (oh, well, we have it already) sometimes allow simpler and more
efficient implementation, since the code need not keep track of pointers.

> 
>>
>>> In that case, cr_hbuf_put() stops doing anything at all because we keep
>>> the memory around.
>>>
>>> cr_hbuf_get() becomes, "I need some memory to write some checkpointy
>>> things into".
>>>
>>> cr_hbuf_put() becomes, "I'm done with this for now, only keep it if
>>> someone else needs it."
>>>
>>> This might all be a lot clearer if we just kept some more explicit
>>> accounting around about who is using the objects.  Something like:
>>>
>>> struct cr_buf {
>>>     struct kref ref;
>>>     int size;
>>>     char buf[0];
>>> };
>>>

[...]

>>
>> Yes, something like that, except you can do without the reference count
>> since the buffer is tied to 'struct cr_ctx'; so this is what I had in
>> mind:
>>
>> In non-buffering mode - as it is now - cr_kwrite() will write the data
>> out.
>>
>> In buffering mode (not implemented yet), cr_write() will either do
>> nothing
>> (if we borrow from the current code, that uses a temp buffer), or attach
>> the 'struct cr_buf' to the 'struct cr_ctx' (if we borrow Dave's
>> suggestion
>> above).
>>
>> In buffering mode, we'll also need 'cr_writeout()' which will write out
>> the entire buffer to the 'struct file'.  (This function will do nothing
>> in non-buffering mode).
>>
>> Finally, the buffers will be freed when the 'struct cr_ctx' is cleaned.
> 
> This smells like a memory pool abstraction and probably shouldn't be
> specific to cr_ctx at all.

Is there something else existing that I can use ?

In Zap I use a chain of large chunks of memory ("large" can even be
MBs each chunk, allocated with vmalloc). cr_hbuf_get() returns pointers
into these chunks. With successive checkpoints, the initial guess as
to the desired total size is based on the last checkpoint. Cleanup is
very fast. Write-out is also fast - write chunks in bulk, not small
buffers one by one. Eventually this allows a faster-than-once-a-second
checkpoint with degrading applications' response time. (of course,
there are other optimizations involved; but improvement was measurable).

> 
> As I mentioned earlier though, you still need to walk the entire
> buffered structure in 'buffered' mode to release COWable object
> references.  Adding a kfree in that path makes the code much easier to
> understand and places the exception/cleanup path in-band rather than
> out-of-band with a memory pool.

For the record, every cr_hbuf_get() is matched by a corresponding
cr_hbuf_put() exactly for this reason - so cleanup is correct even
if we change the implementation of cr_hbuf_... (as suggested by
Dave Hansen).

But actually, why do we need to keep the references to COWable objects
(currently - only memory pages) in the buffer ?
They are kept already on a separate structure (cr_ctx->pgarr_list).
So when we finally write out the data, we first flush the entire buffer
(no memory contents!), followed by the memory contents. The restart
code will be modified as well.

I deliberate avoided all these optimization at this early stage... but
tried to ensure that they can be "easily" added later.

Oren.
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