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https://issues.apache.org/jira/browse/TS-3714?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14615165#comment-14615165
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Sudheer Vinukonda commented on TS-3714:
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Let me clarify the problem and solution to you again. Like I said in my last
comments, the solution already handles the case where nothing's read
immediately following the handshake. So, as such, I don't see it breaking
anything and the solution should be compatible with the master version as well.
OTOH, if you do in fact, see that it's breaking something, let me know and I'll
be happy to fix that.
> TS seems to stall between ua_begin and ua_first_read on some transactions
> resulting in high response times.
> -----------------------------------------------------------------------------------------------------------
>
> Key: TS-3714
> URL: https://issues.apache.org/jira/browse/TS-3714
> Project: Traffic Server
> Issue Type: Bug
> Components: Core
> Affects Versions: 5.3.0
> Reporter: Sudheer Vinukonda
> Assignee: Sudheer Vinukonda
> Fix For: 6.1.0
>
>
> An example slow log showing very high *ua_first_read*.
> {code}
> ERROR: [8624075] Slow Request: client_ip: xx.xx.xx.xxx url:
> http://xxxxxxxxxxxxxxxxxx.... status: 200 unique id: bytes: 86 fd: 0 client
> state: 0 serv
> er state: 9 ua_begin: 0.000 ua_first_read: 42.224 ua_read_header_done: 42.224
> cache_open_rea
> d_begin: 42.224 cache_open_read_end: 42.224 dns_lookup_begin: 42.224
> dns_lookup_end: 42.224
> server_connect: 42.224 server_first_read: 42.229 server_read_header_done:
> 42.229 server_clos
> e: 42.229 ua_begin_write: 42.229 ua_close: 42.229 sm_finish: 42.229
> {code}
> Initially, I suspected that it might be caused by browser's connecting early
> before sending any bytes to TS. However, this seems to be happening too
> frequently and with unrealistically high delay between *ua_begin* and
> *ua_first_read*.
> I suspect it's caused due to the code that disables the read temporarily
> before calling *TXN_START* hook and re-enables it after the API call out. The
> read disable is done via a 0-byte *do_io_read* on the client vc, but, the
> problem is that a valid *mbuf* is still passed. Based on what I am seeing,
> it's possible this results in actually enabling the *read_vio* all the way
> uptil *ssl_read_from_net* for instance (if there's a race condition and there
> were bytes already from the client resulting in an epoll read event), which
> would finally disable the read since, the *ntodo* (nbytes) is 0. However,
> this may result in the epoll event being lost until a new i/o happens from
> the client. I'm trying out further experiments to confirm the theory. In most
> cases, the read buffer already has some bytes by the time the http session
> and http sm is created, which makes it just work. But, if there's a slight
> delay in the receiving bytes after making a connection, the epoll mechanism
> should read it, but, due to the way the temporary read disable is being done,
> the event may be lost (this is coz, ATS uses the epoll edge triggered mode).
> Some history on this issue -
> This issue has been a problem for a long time and affects both http and https
> requests. When this issue was first reported, our router operations team
> eventually closed it indicating that disabling *accept* threads resolved it
> ([~yzlai] also reported similar observations and conclusions). It's possible
> that the race condition window may be slightly reduced by disabling accept
> threads, but, to the overall performance and through put, accept threads are
> very important. I now notice that the issue still exists (regardless of
> whether or not accept threads are enabled/disabled) and am testing further to
> confirm the issue.
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