linux process sleep at socket

I am working on a python2.7 project. I set a process pool with 10 processes. My code structure is:

pool = Pool(10)
pool.map(ProcessJson, jsonFiles)

def ProcessJson(jsonpath):
    # doing something and get (int)numUrls
    for idx in xrange(numUrls):
        flag = DownloadVideo(paras).run()
        if flag == 0:
            continue

class DownloadVideo():
    def __init__(para):
        # init 
    def run(self):
        try:
            videopath = os.path.join(folder, sname.encode("utf-8"))
            try:
                cmd = "wget -q -c --limit-rate=1M --tries=3 -T10 -P %s --output-document=%s \"%s\"" % (folder, videopath, url)
                ret = os.system(cmd)
                if (ret >> 8) != 0:
                    logger.error("cmd error---" + str(ret >> 8) + "---" + cmd + "\n")
                    return 0

            except Exception as e:
                logger.error("python system cmd error---", e)
                return 0

        except Exception as e1:
            logger.error("exception with downloading---", e1)
            return 0

After sometime, the process are all sleep. I use sudo strace -p pid and lsof -p pid to find the problem. What I get are showing:

sudo strace  -p 27723
Process 27723 attached
wait4(-1, 

Then i found its child process 27724

sudo strace  -p 27724
Process 27724 attached
read(6,

cat wchan 27724
do_wait_data

:/proc/27724$ ls -l ./fd

l-wx------ 1 cwz domain^users 64  7月 12 
16:20 3 -> Documents/tools/log/__main__-1.log
lr-x------ 1 cwz domain^users 64  7月 12 
16:20 4 -> pipe:[8180487]
l-wx------ 1 cwz domain^users 64  7月 12 16:20 5 -> 
chenweizhao/Documents/tools/GoogleImages- 
1/Tim_Faraday/xXP4smHFrfLw9M.jpg
lrwx------ 1 cwz domain^users 64  7月 12 16:20 6 -> socket: 
[8286859]
l-wx------ 1 cwz domain^users 64  7月 12 
16:20 7 -> pipe:[8180490]
lr-x------ 1 cwz domain^users 64  7月 12 
16:20 8 -> /dev/null

and

lsof -p 27724
wget    27724 cwz    3w   REG     8,2   427352 5786641 
chenweizhao/tools/log/__main__-1.log
wget    27724 cwz    4r  FIFO     0,8      0t0 8180487 pipe
wget    27724 cwz    5w   REG     8,2    24404 9971506 
chenweizhao/tools/GoogleImages-1/Tim_Faraday/xXP4smHFrfLw9M.jpg
wget    27724 cwz    6u  IPv4 8286859      0t0     
TCP myIP:47496->ec2-13-56-87-172.us-west-1.compute.amazonaws.com:https 
(ESTABLISHED)
wget    27724 cwz    7w  FIFO     0,8      0t0 
8180490 pipe
wget    27724 cwz    8r   CHR     1,3      0t0    1029 
/dev/null

How can I solve the problem? Thank you very much!