减少到服务器/数据库的往返

Question

When writing ASP.NET pages, what signs do you look for that your page is making too many roundtrips to a database or server?

(This is a general question but I say ASP.NET as the majority of my coding is on the web side of things).

Answer 1

How much is too much? The €1M question! Profile. Then profile. If your app is spending most of its time doing data access, you have a problem (and should look at a sql trace). If it is spending most of its time drawing the UI, then (assuming your view isn't doing data access) you should probably look elsewhere first...

Answer 2

Round trips are more relevant to latency than the total quantity of data being moved, so it really does make sense to optimize for them. The usual way is to use stored procedures that do multiple steps, perhaps even returning multiple result sets.

Answer 3

What I do is I look at the ASP performance counters and SQL performance counters. To get an accurate measurement you must ensure that there is no random noise activity on the SQL Server (ie. import batches running unrelated to the web site).

The relevant counters I look at are:

• SQL Statistics/Batch requests/sec : This indicates exactly how many Transact-SQL batches the server receives. It can be, in most cases, equated 1:1 with the number of round trips from the web site to SQL.
• Databases/Transaction/sec : this counter is instanced per database, so I can quickly see in which database there is 'activity'. This way I can correlate the web site data roundtrips (ie. my app logic requests, goes to app database) and the ASP session state and user stuff (goes to Asp session db or tempdb)
• Databases/Write Transaction/sec : This I correlate with the counters above (transaction per second) so I can get a feel of the read-to-write ratio the site is doing.
• ASP.NET Applications/Requests/sec : With this counter I can get the number of requests/sec the site is seeing. Correlated with the number of SQL Batch Requests/sec it gives a good indication of the average number of round-trips per request.

The next thing to measure is usually trying to get a feel for where is the time spent in the request. On my own project, I use abundantly performance counters I publish myself so is really easy to measure. But I'm not always so lucky as to clean up only my own mess... Profiling is usually not an option for me because I most times troubleshoot live production systems I cannot instrument.

My approach is to try to sort out the SQL side of things first, since it's easy to find the relevant statistics for execution times in SQL: SQL keeps a nice aggregated statistic ready to look at in sys.dm_exec_query_stats . I can also use Profiler to measure execution duration in real time. With some analysis of these numbers collected, knowing the normal request pattern of the most visited pages, you can give a pretty good estimate of the total time spent in SQL per web request. If this times adds up to nearly all the time it takes a request to serve the page, then you have your answer.

And to answer the original question title: to reduce the number of round-trips, you make fewer requests. Seriously. First, caching what is appropriate to cache I guess is obvious. Second you reduce the complexity: don't display unnecessary data on each page, you cache and display stale data when you can get away with it, you hide details on secondary navigation panels.

If you feel that the problem is the number of round-trips per se as opposed to the number of requests (ie. you would benefit tremendously from batching multiple requests in one round-trip), then you should somehow measure that the round-trip overhead is what's killing you. With connection pooling on a normal network connection this is usually not the most important factor.

And finally you should look if everything that can be done in sets is done in sets. If you have some half-brained ORM that retrieves objects one at a time from an ID keyset, get rid of it.

Answer 4

I know that this may sound reiterative, but client server round trips depends of how many program logic is located at any side of the connection.

First thing to check is validation: You have to validate and sanitize your input at server side always, but it does not means that you cannot do it too at client side too reducing a round trips that are been used only too check input.

At second: What can you do at client side to reduce server side overload? There are calculations that you can check or make at client side. There is also AJAX that can be used to load only a percentage of the page that is changing.

At third: Can you delegate work to another server? If your server is too loaded, why not to use web services or simply delegate some side of the logic to another server?

As Mark wrote: ¿How is too much? It is is up to you and your budget.

Answer 5

When writing ASP.NET pages, what signs do you look for that your page is making too many roundtrips to a database or server?

Of course it all depends and you have to profile. However, here are some indicators, they do not mean there is a problem, but often will indicate

• Page is taking a very long time to render locally.

  • Read this question: Slow response-time cheat sheet , In particular this link

• To render the page you need more than 30 round trips. I pulled that number out of my hat, but assuming a round trip is taking about 3.5ms then 30 round trips will kick you over the 100ms guideline (before any other kind of processing).

• All the queries involved in rendering the page are heavily optimized and do not take longer than a millisecond or two to execute. There are no operations that require lots of CPU cycles that execute every time you render the page.

• Data access is abstracted away and not cached in any kind of way. If, for example, GetCustomer will call the DAL which in turn issues a query and your page is asking for 100 Customer objects which are not retrieved in a batch, you are probably in trouble.