Re: [SPDK] NBD with SPDK

[Harris, James R <james.r.harris at intel.com>]

[-- Attachment #1: Type: text/plain, Size: 4308 bytes --]



On 8/12/19, 9:20 AM, "SPDK on behalf of Mittal, Rishabh via SPDK" <spdk-bounces(a)lists.01.org on behalf of spdk(a)lists.01.org> wrote:

    <<As I’m sure you’re aware, SPDK apps use spdk_alloc() with the SPDK_MALLOC_DMA which is backed by huge pages that are effectively pinned already. SPDK does virt to phy transition on memory allocated this <<way very efficiently using spdk_vtophys().  It would be an interesting experiment though.  Your app is not in a VM right?>>
    
    I am thinking of passing the physical address of the buffers in bio to spdk.  I don’t know if it is already pinned by the kernel or do we need to explicitly do it. And also, spdk has some requirements on the alignment of physical address. I don’t know if address in bio conforms to those requirements.
    
    SPDK won’t be running in VM.


Hi Rishabh,

SPDK relies on data buffers being mapped into the SPDK application's address space, and are passed as virtual addresses throughout the SPDK stack.  Once the buffer reaches a module that requires a physical address (such as the NVMe driver for a PCIe-attached device), SPDK translates the virtual address to a physical address.  Note that the NVMe fabrics transports (RDMA and TCP) both deal with virtual addresses, not physical addresses.  The RDMA transport is built on top of ibverbs, where we register virtual address areas as memory regions for describing data transfers.

So for nbd, pinning the buffers and getting the physical address(es) to SPDK wouldn't be enough.  Those physical address regions would also need to get dynamically mapped into the SPDK address space.

Do you have any profiling data that shows the relative cost of the data copy v. the system calls themselves on your system?  There may be some optimization opportunities on the system calls to look at as well.

Regards,

-Jim




    From: "Luse, Paul E" <paul.e.luse(a)intel.com>
    Date: Sunday, August 11, 2019 at 12:53 PM
    To: "Mittal, Rishabh" <rimittal(a)ebay.com>, "spdk(a)lists.01.org" <spdk(a)lists.01.org>
    Cc: "Kadayam, Hari" <hkadayam(a)ebay.com>, "Chen, Xiaoxi" <xiaoxchen(a)ebay.com>, "Szmyd, Brian" <bszmyd(a)ebay.com>
    Subject: RE: NBD with SPDK
    
    Hi Rishabh,
    
    Thanks for the question. I was talking to Jim and Ben about this a bit, one of them may want to elaborate but we’re thinking the cost of mmap and also making sure the memory is pinned is probably prohibitive. As I’m sure you’re aware, SPDK apps use spdk_alloc() with the SPDK_MALLOC_DMA which is backed by huge pages that are effectively pinned already. SPDK does virt to phy transition on memory allocated this way very efficiently using spdk_vtophys().  It would be an interesting experiment though.  Your app is not in a VM right?
    
    Thx
    Paul
    
    From: Mittal, Rishabh [mailto:rimittal(a)ebay.com]
    Sent: Saturday, August 10, 2019 6:09 PM
    To: spdk(a)lists.01.org
    Cc: Luse, Paul E <paul.e.luse(a)intel.com>; Kadayam, Hari <hkadayam(a)ebay.com>; Chen, Xiaoxi <xiaoxchen(a)ebay.com>; Szmyd, Brian <bszmyd(a)ebay.com>
    Subject: NBD with SPDK
    
    Hi,
    
    We are trying to use NBD and SPDK on client side.  Data path looks like this
    
    File System ----> NBD client ------>SPDK------->NVMEoF
    
    
    Currently we are seeing a high latency in the order of 50 us by using this path. It seems like there is data buffer copy happening for write commands from kernel to user space when spdk nbd read data from the nbd socket.
    
    I think that there could be two ways to prevent data copy .
    
    
      1.  Memory mapped the kernel buffers to spdk virtual space.  I am not sure if it is possible to mmap a buffer. And what is the impact to call mmap for each IO.
      2.  If NBD kernel give the physical address of a buffer and SPDK use that to DMA it to NVMEoF. I think spdk must also be changing a virtual address to physical address before sending it to nvmeof.
    
    Option 2 makes more sense to me. Please let me know if option 2 is feasible in spdk
    
    Thanks
    Rishabh Mittal
    
    _______________________________________________
    SPDK mailing list
    SPDK(a)lists.01.org
    https://lists.01.org/mailman/listinfo/spdk