On Mon, Apr 27, 2020 at 5:10 PM Liam <networkimp...@gmail.com> wrote: > > On Monday, April 27, 2020 at 4:22:41 PM UTC-7, Ian Lance Taylor wrote: >> >> On Sun, Apr 26, 2020 at 4:55 PM Liam <networ...@gmail.com> wrote: >> > >> > During an io.Copy() where the Writer is a TCPConn and the Reader is a 200K >> > disk file, my code may concurrently Write() on the same TCPConn. >> > >> > I see the result of the Write() inserted into the result of the io.Copy(). >> > I had the impression that was impossible, but I must be mistaken, as the >> > sendfile(2) docs read: >> > >> > Note that a successful call to sendfile() may write fewer bytes than >> > requested; the caller should be prepared to retry the call if there were >> > unsent bytes. >> > >> > Could someone confirm that one must indeed synchronize concurrent use of >> > tcpConn.Write() and io.Copy(tcpConn, file)? >> >> Synchronization should not be required. internal/poll.Sendfile >> acquires a write lock on dstFD, which is the TCP socket. That should >> ensure that the contents of an ordinary Write (which also acquires a >> write lock) should not interleave with the sendfile data. >> >> That said, if the sendfile system call cannot be used for whatever >> reason, the net package will fall back on doing ordinary Read and >> Write calls. And those Write calls can be interleaved with other >> Write calls done by a different goroutine. I think that is probably >> permitted, in that io.Copy doesn't promise to not interleave with >> simultaneous Write calls on the destination. >> >> So in the general case you should indeed use your own locking to avoid >> interleaving between io.Copy and a concurrent Write. > > > Thanks for the details. Where could I add a Println() to reveal why it > doesn't call poll.Sendfile()? > > I expect this system to use sendfile(2). The file is a normal file on a local > partition (running on a Digital Ocean Droplet). > > > /etc/fstab has: > UUID=[omitted] / ext4 defaults 1 1 > > > $ df -h > Filesystem Size Used Avail Use% Mounted on > devtmpfs 981M 0 981M 0% /dev > tmpfs 996M 0 996M 0% /dev/shm > tmpfs 996M 436K 995M 1% /run > tmpfs 996M 0 996M 0% /sys/fs/cgroup > /dev/vda1 59G 5.7G 51G 11% / > tmpfs 200M 0 200M 0% /run/user/0
Well, I don't know for that it doesn't use sendfile, I just can't explain the results you're seeing if it does use sendfile. The place to start is to find out why (or whether) internal/poll.Sendfile is returning 0, nil. Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcU5i0Z-N3R%3D6ozMCc3EQUz%3DTud-8tNDNM2nJDdR6JdXQA%40mail.gmail.com.
