Re: [Qemu-devel] [PATCH v2] hw/net/opencores_eth: Allocating Large sized arrays to heap

[Zhou Jie <zhoujie2011@cn.fujitsu.com>]

Hi Max,

    When I committed another patch which named as
    "hw/net/virtio-net: Allocating Large sized arrays to heap" .

    Christian Borntraeger said that 16k is usually perfectly fine
    for a userspace stack and doing allocations in a hot path
    might actually hurt performance.

    Although the size is 65536 bytes here,
    I think open_eth_start_xmit is in a hot path.
    So, it is OK, if you think that this patch should not be applied.

Sincerely,
Zhou Jie

On 2016/4/27 10:07, Zhou Jie wrote:
> open_eth_start_xmit has a huge stack usage of 65536 bytes approx.
> Moving large arrays to heap to reduce stack usage.
>
> Signed-off-by: Zhou Jie <zhoujie2011@cn.fujitsu.com>
> ---
>   hw/net/opencores_eth.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/hw/net/opencores_eth.c b/hw/net/opencores_eth.c
> index c6094fb..fa0a4e7 100644
> --- a/hw/net/opencores_eth.c
> +++ b/hw/net/opencores_eth.c
> @@ -483,7 +483,8 @@ static NetClientInfo net_open_eth_info = {
>
>   static void open_eth_start_xmit(OpenEthState *s, desc *tx)
>   {
> -    uint8_t buf[65536];
> +    uint8_t *buf = NULL;
> +    uint8_t buffer[0x600];
>       unsigned len = GET_FIELD(tx->len_flags, TXD_LEN);
>       unsigned tx_len = len;
>
> @@ -498,6 +499,11 @@ static void open_eth_start_xmit(OpenEthState *s, desc *tx)
>
>       trace_open_eth_start_xmit(tx->buf_ptr, len, tx_len);
>
> +    if (tx_len > 0x600) {
> +        buf = g_new(uint8_t, tx_len);
> +    } else {
> +        buf = buffer;
> +    }
>       if (len > tx_len) {
>           len = tx_len;
>       }
> @@ -506,6 +512,9 @@ static void open_eth_start_xmit(OpenEthState *s, desc *tx)
>           memset(buf + len, 0, tx_len - len);
>       }
>       qemu_send_packet(qemu_get_queue(s->nic), buf, tx_len);
> +    if (tx_len > 0x600) {
> +        g_free(buf);
> +    }
>
>       if (tx->len_flags & TXD_WR) {
>           s->tx_desc = 0;
>

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