- There are two cases where the server can potentially block for a long
time while servicing a request: when handling a yp_all() request, which
could take a while to complete if the map being transfered is large
(e.g. 'ypcat passwd' where passwd.byname has 10,000 entries in it),
and while doing DNS lookups when in SunOS compat mode (with the -dns
flag), since some DNS lookups can take a long time to complete. While
ypserv is blocked, other clients making requests to the server will
also block. To fix this, we fork() ypall and DNS lookups into subprocesses
and let the parent ypserv process go on servicing other incoming
requests.
We place a cap on the number of simultaneous processes that ypserv can
fork (set at 20 for now) and go back to 'linear mode' if it hits the
limit (which just means it won't fork() anymore until the number of
simultaneous processes drops under 20 again). The cap does not apply
to fork()s done as a result of ypxfr calls, since we want to do our
best to insure that map transfers from master servers succeed.
To make this work, we need our own special copy of svc_run() so that
we can properly terminate child processes once the RPC dispatch
functions have run.
(I have no idea what SunOS does in this situation. The only other
possibility I can think of is async socket I/O, but that seems
like a headache and a half to implement.)
- Do the politically correct thing and use sigaction() instead of
signal() to install the SIGCHLD handler and to ignore SIGPIPEs.
- Doing a yp_all() is sometimes slow due to the way read_database() is
implemented. This is turn is due to a certain deficiency in the DB
hash method: the R_CURSOR flag doesn't work, which means that when
handed a key and asked to return the key/data pair for the _next_
key in the map, we have to reset the DB pointer to the start of the
database, step through until we find the requested key, step one
space ahead to the _next_ key, and then use that. (The original ypserv
code used GDBM has a function called gdbm_nextkey() that does
this for you.) This can get really slow for large maps. However,
when doing a ypall, it seems that all database access are sequential,
so we can forgo the first step (the 'search the database until we find
the key') since the database should remain open and the cursor
should be positioned at the right place until the yp_all() call
finishes. We can't make this assumption for arbitrary yp_first()s
and yp_next()s however (since we may have requests from several clients
for different maps all arriving at different times) so those we have
to handle the old way.
(This would be much easier if R_CURSOR really worked. Maybe I should
be using something other than the hash method.)
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