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SubjectRe: [PATCH] mlx4_ib: Increase the timeout for CM cache
On Thu, Jan 31, 2019 at 06:09:51PM +0100, Håkon Bugge wrote:
> Using CX-3 virtual functions, either from a bare-metal machine or
> pass-through from a VM, MAD packets are proxied through the PF driver.
>
> Since the VMs have separate name spaces for MAD Transaction Ids
> (TIDs), the PF driver has to re-map the TIDs and keep the book keeping
> in a cache.
>
> Following the RDMA CM protocol, it is clear when an entry has to
> evicted form the cache. But life is not perfect, remote peers may die
> or be rebooted. Hence, it's a timeout to wipe out a cache entry, when
> the PF driver assumes the remote peer has gone.
>
> We have experienced excessive amount of DREQ retries during fail-over
> testing, when running with eight VMs per database server.
>
> The problem has been reproduced in a bare-metal system using one VM
> per physical node. In this environment, running 256 processes in each
> VM, each process uses RDMA CM to create an RC QP between himself and
> all (256) remote processes. All in all 16K QPs.
>
> When tearing down these 16K QPs, excessive DREQ retries (and
> duplicates) are observed. With some cat/paste/awk wizardry on the
> infiniband_cm sysfs, we observe:
>
> dreq: 5007
> cm_rx_msgs:
> drep: 3838
> dreq: 13018
> rep: 8128
> req: 8256
> rtu: 8256
> cm_tx_msgs:
> drep: 8011
> dreq: 68856
> rep: 8256
> req: 8128
> rtu: 8128
> cm_tx_retries:
> dreq: 60483
>
> Note that the active/passive side is distributed.
>
> Enabling pr_debug in cm.c gives tons of:
>
> [171778.814239] <mlx4_ib> mlx4_ib_multiplex_cm_handler: id{slave:
> 1,sl_cm_id: 0xd393089f} is NULL!
>
> By increasing the CM_CLEANUP_CACHE_TIMEOUT from 5 to 30 seconds, the
> tear-down phase of the application is reduced from 113 to 67
> seconds. Retries/duplicates are also significantly reduced:
>
> cm_rx_duplicates:
> dreq: 7726
> []
> cm_tx_retries:
> drep: 1
> dreq: 7779
>
> Increasing the timeout further didn't help, as these duplicates and
> retries stem from a too short CMA timeout, which was 20 (~4 seconds)
> on the systems. By increasing the CMA timeout to 22 (~17 seconds), the
> numbers fell down to about one hundred for both of them.
>
> Adjustment of the CMA timeout is _not_ part of this commit.
>
> Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
> ---
> drivers/infiniband/hw/mlx4/cm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)

Jack? What do you think?

> diff --git a/drivers/infiniband/hw/mlx4/cm.c b/drivers/infiniband/hw/mlx4/cm.c
> index fedaf8260105..8c79a480f2b7 100644
> --- a/drivers/infiniband/hw/mlx4/cm.c
> +++ b/drivers/infiniband/hw/mlx4/cm.c
> @@ -39,7 +39,7 @@
>
> #include "mlx4_ib.h"
>
> -#define CM_CLEANUP_CACHE_TIMEOUT (5 * HZ)
> +#define CM_CLEANUP_CACHE_TIMEOUT (30 * HZ)
>
> struct id_map_entry {
> struct rb_node node;
> --
> 2.20.1
>

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