Re: [Qemu-trivial] [Qemu-devel] [PATCH] net/tap: Allocating Large sized arrays to heap

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

On 2016/4/26 15:45, Christian Borntraeger wrote:
> On 04/26/2016 03:26 AM, Zhou Jie wrote:
>> net_init_tap has a huge stack usage of 8192 bytes approx.
>> Moving large arrays to heap to reduce stack usage.
>
> I am wondering. Why is 8k a problem for a user space program?
For 64bit machine it will be 16k.

> Please note that malloc/new like allocations are much more expensive
> than stack allocation in terms of performance. This does not matter
> here, but in your other patch that deals with the xmit function, I would
> not be surprised if that actually harms performance.
OK. I will note it.

Sincerely,
Zhou Jie

>
> Christian
>
>
>
>>
>> Signed-off-by: Zhou Jie <zhoujie2011@cn.fujitsu.com>
>> ---
>>   net/tap.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/tap.c b/net/tap.c
>> index 740e8a2..49817c7 100644
>> --- a/net/tap.c
>> +++ b/net/tap.c
>> @@ -769,8 +769,8 @@ int net_init_tap(const NetClientOptions *opts, const char *name,
>>               return -1;
>>           }
>>       } else if (tap->has_fds) {
>> -        char *fds[MAX_TAP_QUEUES];
>> -        char *vhost_fds[MAX_TAP_QUEUES];
>> +        char **fds = g_new(char *, MAX_TAP_QUEUES);
>> +        char **vhost_fds = g_new(char *, MAX_TAP_QUEUES);
>>           int nfds, nvhosts;
>>
>>           if (tap->has_ifname || tap->has_script || tap->has_downscript ||
>> @@ -818,6 +818,8 @@ int net_init_tap(const NetClientOptions *opts, const char *name,
>>                   return -1;
>>               }
>>           }
>> +        g_free(fds);
>> +        g_free(vhost_fds);
>>       } else if (tap->has_helper) {
>>           if (tap->has_ifname || tap->has_script || tap->has_downscript ||
>>               tap->has_vnet_hdr || tap->has_queues || tap->has_vhostfds) {
>>
>
>
>
>

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