On 20-Mar-18 12:42 PM, Olivier Matz wrote:
On Tue, Mar 20, 2018 at 10:27:55AM +0000, Burakov, Anatoly wrote:
On 19-Mar-18 5:30 PM, Olivier Matz wrote:
Hi Anatoly,
On Sat, Mar 03, 2018 at 01:45:48PM +0000, Anatoly Burakov wrote:
This patchset introduces dynamic memory allocation for DPDK (aka memory
hotplug). Based upon RFC submitted in December [1].
Dependencies (to be applied in specified order):
- IPC bugfixes patchset [2]
- IPC improvements patchset [3]
- IPC asynchronous request API patch [4]
- Function to return number of sockets [5]
Deprecation notices relevant to this patchset:
- General outline of memory hotplug changes [6]
- EAL NUMA node count changes [7]
The vast majority of changes are in the EAL and malloc, the external API
disruption is minimal: a new set of API's are added for contiguous memory
allocation for rte_memzone, and a few API additions in rte_memory due to
switch to memseg_lists as opposed to memsegs. Every other API change is
internal to EAL, and all of the memory allocation/freeing is handled
through rte_malloc, with no externally visible API changes.
Quick outline of all changes done as part of this patchset:
* Malloc heap adjusted to handle holes in address space
* Single memseg list replaced by multiple memseg lists
* VA space for hugepages is preallocated in advance
* Added alloc/free for pages happening as needed on rte_malloc/rte_free
* Added contiguous memory allocation API's for rte_memzone
* Integrated Pawel Wodkowski's patch for registering/unregistering memory
with VFIO [8]
* Callbacks for registering memory allocations
* Multiprocess support done via DPDK IPC introduced in 18.02
The biggest difference is a "memseg" now represents a single page (as opposed to
being a big contiguous block of pages). As a consequence, both memzones and
malloc elements are no longer guaranteed to be physically contiguous, unless
the user asks for it at reserve time. To preserve whatever functionality that
was dependent on previous behavior, a legacy memory option is also provided,
however it is expected (or perhaps vainly hoped) to be temporary solution.
Why multiple memseg lists instead of one? Since memseg is a single page now,
the list of memsegs will get quite big, and we need to locate pages somehow
when we allocate and free them. We could of course just walk the list and
allocate one contiguous chunk of VA space for memsegs, but this
implementation uses separate lists instead in order to speed up many
operations with memseg lists.
For v1, the following limitations are present:
- FreeBSD does not even compile, let alone run
- No 32-bit support
- There are some minor quality-of-life improvements planned that aren't
ready yet and will be part of v2
- VFIO support is only smoke-tested (but is expected to work), VFIO support
with secondary processes is not tested; work is ongoing to validate VFIO
for all use cases
- Dynamic mapping/unmapping memory with VFIO is not supported in sPAPR
IOMMU mode - help from sPAPR maintainers requested
Nevertheless, this patchset should be testable under 64-bit Linux, and
should work for all use cases bar those mentioned above.
[1] http://dpdk.org/dev/patchwork/bundle/aburakov/Memory_RFC/
[2] http://dpdk.org/dev/patchwork/bundle/aburakov/IPC_Fixes/
[3] http://dpdk.org/dev/patchwork/bundle/aburakov/IPC_Improvements/
[4] http://dpdk.org/dev/patchwork/bundle/aburakov/IPC_Async_Request/
[5] http://dpdk.org/dev/patchwork/bundle/aburakov/Num_Sockets/
[6] http://dpdk.org/dev/patchwork/patch/34002/
[7] http://dpdk.org/dev/patchwork/patch/33853/
[8] http://dpdk.org/dev/patchwork/patch/24484/
I did a quick pass on your patches (unfortunately, I don't have
the time to really dive in it).
I have few questions/comments:
- This is really a big patchset. Thank you for working on this topic.
I'll try to test our application with it as soon as possible.
- I see from patch 17 that it is possible that rte_malloc() expands
the heap by requesting more memory to the OS? Did I understand well?
Today, a good property of rte_malloc() compared to malloc() is that
it won't interrupt the process (the worst case is a spinlock). This
is appreciable on a dataplane core. Will it change?
Hi Olivier,
Not sure what you mean by "interrupt the process". The new rte_malloc will
_mostly_ work just like the old one. There are now two levels of locks: the
heap lock, and the system allocation lock. If your rte_malloc call requests
amount of memory that can be satisfied by already allocated memory, then
only the heap lock is engaged - or, to put it in other words, things work as
before.
When you *don't* have enough memory allocated, previously rte_malloc would
just fail. Now, it instead will lock the second lock and try to allocate
more memory from the system. This requires IPC (to ensure all processes have
allocated/freed the same memory), so this will take way longer (timeout is
set to wait up to 5 seconds, although under normal circumstances it's taking
a lot less - depending on how many processes you have running, but generally
under 100ms), and will block other system allocations (i.e. if another
rte_malloc call on another heap is trying to request more memory from the
system).
So, in short - you can't allocate from the same heap in parallel (same as
before), and you can't have parallel system memory allocation requests
(regardless of from which heap it comes from). The latter *only* applies to
system memory allocations - that is, if one heap is allocating system memory
while another heap receives allocation request *and is able to satisfy it
from already allocated memory*, it will not block, because the second lock
is never engaged.
OK. Let's imagine you are using rte_malloc() on a dataplane core, and
you run out of memory. Previously, the allocation would just fail. Now,
if my understanding is correct, it can block for a long time, which can
be a problem on a dataplane core, because it will cause packet losses,
especially if it also blocks allocations on other cores during that
time. In this case, it could be useful to make the dynamic heap resizing
feature optional.
Why would anyone in their right mind call rte_malloc on fast path? If
you're referring to mempool allocations/deallocations, then this is a
completely separate subject, as mempool alloc/free is not handled by
rte_malloc but is handled by rte_mempool itself - as far as rte_malloc
is concerned, that memory is already allocated and it will not touch it.
As for "making heap resizing feature optional", i'm working on
functionality that would essentially enable that. Specifically, i'm
adding API's to set allocation limits and a callback which will get
triggered once allocator tries to allocate beyond said limits, with an
option of returning -1 and thus preventing this allocation from
completing. While this is kind of a round-about way of doing it, it
would have similar effect.
I have another question about the patchset. Today, it is not really
possible for an application to allocate a page. If you want a full page
(ex: 2M), you need to allocate 4M because the rte_malloc layer adds a
header before the allocated memory. Therefore, if the memory is
fragmented a lot with only 2M pages, you cannot allocate them as pages.
It is possible, with your patchset or in the future, to have an access
to a page-based allocator? The use-case is to be able for an application
to ask for pages in dpdk memory and remap them in a virtually contiguous
memory.
Pages returned from our allocator are already virtually contiguous,
there is no need to do any remapping. If user specifies proper size and
alignment (i.e. reserve a memzone with RTE_MEMZONE_2MB and with 2M size
and alignment), it will essentially cause the allocator to return a
memzone that's exactly page-size long. Yes, in the background, it will
allocate another page to store malloc metadata, and yes, memory will
become fragmented if multiple such allocations will occur. It is not
possible (neither now nor in the future planned work) to do what you
describe unless we store malloc data separately from allocated memory
(which can be done, but is a non-trivial amount of work).
Malloc stores its metadata right in the hugepage mostly for multiprocess
purposes - so that the entire heap is always shared between all
processes. If we want to store malloc metadata separately from allocated
memory, a replacement mechanism to shared heap metadata will need to be
put in place (which, again, can be done, but is a non-trivial amount of
work - arguably for questionable gain).
That said, use case you have described is already possible - just
allocate multiple pages from DPDK as a memzone, and overlay your own
memory allocator over that memory. This will have the same effect.
Thanks
Olivier
--
Thanks,
Anatoly
