netdev - mlx4 interrupt coalescing not important for small packets?

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Message-ID: <20130809170637.GA5258@sbohrermbp13-local.rgmadvisors.com>
Date:	Fri, 9 Aug 2013 12:06:37 -0500
From:	Shawn Bohrer <shawn.bohrer@...il.com>
To:	netdev <netdev@...r.kernel.org>
Cc:	Amir Vadai <amirv@...lanox.com>
Subject: mlx4 interrupt coalescing not important for small packets?

I was looking at the adaptive interrupt coalescing algorithm for
mlx4_en which contains the following snippet:

/* Apply auto-moderation only when packet rate
 * exceeds a rate that it matters */
if (rate > (MLX4_EN_RX_RATE_THRESH / priv->rx_ring_num) &&
    avg_pkt_size > MLX4_EN_AVG_PKT_SMALL) {
        if (rate < priv->pkt_rate_low)
                moder_time = priv->rx_usecs_low;
        else if (rate > priv->pkt_rate_high)
                moder_time = priv->rx_usecs_high;
        else
                moder_time = (rate - priv->pkt_rate_low) *
                        (priv->rx_usecs_high - priv->rx_usecs_low) /
                        (priv->pkt_rate_high - priv->pkt_rate_low) +
                        priv->rx_usecs_low;
} else {
        moder_time = priv->rx_usecs_low;
}

In this case MLX4_EN_AVG_PKT_SMALL is 256 bytes.  Can someone explain
to me why interrupt coalescing is not important for small packets
regardless of the incoming packet rate?

Thanks,
Shawn
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