[ofiwg] Hugepages usage in libfabric

James Swaro jswaro at cray.com
Wed Apr 10 08:39:37 PDT 2019

From the ewg mailing list entry:

> Changes since v3:
> - determining the page size of a given memory range by watching madvise()
>   fail has proven to be unreliable. So we introduce the RDMAV_HUGEPAGES_SAFE
>   environment variable to let the user decide if the page size should be
>   checked on every reg_mr() call or not. This requires the user to be aware
>   if huge pages are used by the running application or not.
> I did not add an aditional API call to enable this, as applications can use
> setenv() + ibv_fork_init() to enable checking for huge pages in the code.

It looks like a conscious decision was made to force applications to call ibv_fork_init if they wanted hugepages support, regardless of intent to fork.

-- Jim

On 4/10/19, 10:32 AM, "ofiwg on behalf of James Swaro" <ofiwg-bounces at lists.openfabrics.org on behalf of jswaro at cray.com> wrote:

    I'm not certain what prompted performing the check in ibv_fork_init. However, it seems to me that hugepages registration support is orthogonal to forking. Given what the environment variable addresses and the fact that the variable is only read in ibv_fork_init, I would wonder if the application would crash if it did NOT call ibv_fork_init -- even if it wasn't planning to fork. 
    James Swaro 
    Cray Inc. 
    On 4/10/19, 10:03 AM, "Jason Gunthorpe" <jgg at ziepe.ca> wrote:
        On Wed, Apr 10, 2019 at 02:36:53PM +0000, James Swaro wrote:
        >    I’d be interested in exposing an environment variable, flag, or general
        >    tunable in libfabric for applications to indicate whether they want to
        >    use huge pages. Using the verbs provider on an internal development
        >    system, I’ve run into an issue where use of huge pages makes the memory
        >    registration function fail unless RDMAV_HUGEPAGES_SAFE is set.
        >    See [1]https://lists.openfabrics.org/pipermail/ewg/2010-July/015609.htm
        >    l for context.
        Oh gross, this should just be fixed in verbs to not require the ugly
        environment variable in the first place.
        >    general, if necessary. For verbs, use of the RDMAV_HUGEPAGES_SAFE
        >    variable tends to impact performance mostly after the rendezvous
        >    threshold. After reaching the rendezvous threshold, latency for data
        >    transfer operations appears to increase by a factor of ten.
        If the app isn't forking it shouldn't even be calling ibv_fork_init in
        the first place, which solves both problems.
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